Three-dimensional vertical NOR flash thin-film transistor strings

ABSTRACT

A memory structure, includes active columns of polysilicon formed above a semiconductor substrate, each active column includes one or more vertical NOR strings, with each NOR string having thin-film storage transistors sharing a local source line and a local bit line, the local bit line is connected by one segment of a segmented global bit line to a sense amplifier provided in the semiconductor substrate.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patentapplication Ser. No. 16/447,406, entitled “Three-dimensional verticalNOR Flash Thin-Film Transistor Strings,” filed Jun. 20, 2019, which is acontinuation application of U.S. patent application Ser. No. 16/252,301,entitled “Three-dimensional vertical NOR Flash Thin-Film TransistorStrings,” filed Jan. 18, 2019, which is a continuation-in-partapplication of copending U.S. patent application (“Non-ProvisionalApplication I”), Ser. No. 16/107,732, entitled “Three-dimensionalvertical NOR Flash Thin-Film Transistor Strings,” filed on Aug. 21,2018, which is a continuation application of U.S. patent applicationSer. No. 15/837,734 entitled “Three-dimensional vertical NOR FlashThin-Film Transistor Strings,” filed on Dec. 11, 2017, now U.S. Pat. No.10,096,364, which is a divisional application of U.S. patent applicationSer. No. 15/343,332, entitled “Three-dimensional vertical NOR FlashThin-Film Transistor Strings,” filed on Nov. 4, 2016, now U.S. Pat. No.9,842,651, which claims priority of (i) U.S. provisional patentapplication (“Provisional Application I”), Ser. No. 62/260,137, entitled“Three-dimensional Vertical NOR Flash Thin-film Transistor Strings,”filed on Nov. 25, 2015; (ii) copending U.S. non-provisional patentapplication (“Non-Provisional Application II”), Ser. No. 15/220,375,entitled “Multi-Gate NOR Flash Thin-film Transistor Strings Arranged inStacked Horizontal Active Strips With Vertical Control Gates,” filed onJul. 26, 2016, now U.S. Pat. No. 9,892,800; and (iii) U.S. provisionalpatent application (“Provisional Application II”), Ser. No. 62/363,189,entitled “Capacitive Coupled Non-Volatile Thin-film Transistor Strings,”filed Jul. 15, 2016; and (iv) copending U.S. non-provisional patentapplication (“Non-Provisional Patent Application III”), Ser. No.15/248,420, entitled “Capacitive Coupled Non-Volatile Thin-filmTransistor Strings in Three-Dimensional Array,” filed Aug. 26, 2016.

The present application is also related to and claims priority of U.S.provisional application (“Provisional Application III”), Ser. No.62/625,818, entitled “Three-dimensional vertical NOR Flash Thin-FilmTransistor Strings,” filed on Feb. 2, 2018 and U.S. provisional patentapplication (“Provisional Application IV”), Ser. No. 62/630,214,entitled “Three-dimensional vertical NOR Flash Thin-Film TransistorStrings,” filed on Feb. 13, 2018.

The disclosures of Provisional Applications I-IV and Non-ProvisionalApplication I-III are hereby incorporated by reference in theirentireties.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to high-density memory structures. Inparticular, the present invention relates to high-density memorystructures formed by interconnected thin film storage elements, such asthin film storage transistors formed in vertical strips with horizontalword lines.

2. Discussion of the Related Art

In this disclosure, memory circuit structures are described. Thesestructures may be fabricated on planar semiconductor substrates (e.g.,silicon wafers) using conventional fabrication processes. To facilitateclarity in this description, the term “vertical” refers to the directionperpendicular to the surface of a semiconductor substrate, and the term“horizontal” refers to any direction that is parallel to the surface ofthat semiconductor substrate.

A number of high-density non-volatile memory structures, such as“three-dimensional vertical NAND strings,” are known in the prior art.Many of these high-density memory structures are formed using thin filmstorage transistors formed out of deposited thin films (e.g.,polysilicon thin films), and organized as arrays of “memory strings.”One type of memory strings is referred to as NAND memory strings orsimply “NAND strings”. A NAND string consists of a number ofseries-connected thin film storage transistors (“TFTs”). Reading orprogramming the content of any of the series-connected TFTs requiresactivation of all series-connected TFTs in the string. Thin film NANDtransistors have lower conductivity than NAND transistors formed insingle crystal silicon, therefore the low read current that is requiredto be conducted through a long string of NAND TFTs results in arelatively slow read access (i.e. long latency).

Another type of high density memory structures is referred to as the NORmemory strings or “NOR strings.” A NOR string includes a number ofstorage transistors each of which is connected to a shared source regionand a shared drain region. Thus, the transistors in a NOR string areconnected in parallel, so that a read current in a NOR string isconducted over a much lesser resistance than the read current through aNAND string. To read or program a storage transistor in a NOR string,only that storage transistor needs to be activated (i.e., “on” orconducting), all other storage transistors in the NOR string may remaindormant (i.e., “off” or non-conducting). Consequently, a NOR stringallows much faster sensing of the activated storage transistor to beread. Conventional NOR transistors are programmed by a channelhot-electron injection technique, in which electrons are accelerated inthe channel region by a voltage difference between the source region andthe drain region and are injected into the charge-trapping layer betweenthe control gate and the channel region, when an appropriate voltage isapplied to the control gate. Channel hot-electron injection programmingrequires a relatively large electron current to flow through the channelregion, therefore limiting the number of transistors that can beprogrammed in parallel. Unlike transistors that are programmed byhot-electron injection, in transistors that are programmed byFowler-Nordheim tunneling or by direct tunneling, electrons are injectedfrom the channel region to the charge-trapping layer by a high electricfield that is applied between the control gate and the source and drainregions. Fowler-Nordheim tunneling and direct tunneling are orders ofmagnitude more efficient than channel hot-electron injection, allowingmassively parallel programming; however, such tunneling is moresusceptible to program-disturb conditions.

3-Dimensional NOR memory arrays are disclosed in U.S. Pat. No. 8,630,114to H. T Lue, entitled “Memory Architecture of 3D NOR Array”, filed onMar. 11, 2011 and issued on Jan. 14, 2014.

U.S. patent Application Publication US2016/0086970 A1 by Haibing Peng,entitled “Three-Dimensional Non-Volatile NOR-type Flash Memory,” filedon Sep. 21, 2015 and published on Mar. 24, 2016, discloses non-volatileNOR flash memory devices consisting of arrays of basic NOR memory groupsin which individual memory cells are stacked along a horizontaldirection parallel to the semiconductor substrate with source and drainelectrodes shared by all field effect transistors located at one or twoopposite sides of the conduction channel.

Three-dimensional vertical memory structures are disclosed, for example,in U.S. Pat. No. 8,878,278 to Alsmeier et al. (“Alsmeier”), entitled“Compact Three-Dimensional Vertical NAND and Methods of Making Thereof,”filed on Jan. 30, 2013 and issued on Nov. 4, 2014. Alsmeier disclosesvarious types of high-density NAND memory structures, such as “terabitcell array transistor” (TCAT) NAND arrays (FIG. 1A), “pipe-shapedbit-cost scalable” (P-BiCS) flash memory (FIG. 1B) and a “vertical NAND”memory string structure. Likewise, U.S. Pat. No. 7,005,350 to Walker etal. (“Walker I”), entitled “Method for Fabricating Programmable MemoryArray Structures Incorporating Series—Connected Transistor Strings,”filed on Dec. 31, 2002 and issued on Feb. 28, 2006, also discloses anumber of three-dimensional high-density NAND memory structures.

U.S. Pat. No. 7,612,411 to Walker (“Walker II”), entitled “Dual-GateDevice and Method” filed on Aug. 3, 2005 and issued on Nov. 3, 2009,discloses a “dual gate” memory structure, in which a common activeregion serves independently controlled storage elements in two NANDstrings formed on opposite sides of the common active region.

3-Dimensional NOR memory arrays are disclosed in U.S. Pat. No. 8,630,114to H. T Lue, entitled “Memory Architecture of 3D NOR Array,” filed onMar. 11, 2011 and issued on Jan. 14, 2014.

A three-dimensional memory structure, including horizontal NAND stringsthat are controlled by vertical polysilicon gates, is disclosed in thearticle “Multi-layered Vertical gate NAND Flash Overcoming StackingLimit for Terabit Density Storage” (“Kim”), by W. Kim et al., publishedin the 2009 Symposium on VLSI Tech. Dig. of Technical Papers, pp188-189. Another three-dimensional memory structure, also includinghorizontal NAND strings with vertical polysilicon gates, is disclosed inthe article, “A Highly Scalable 8-Layer 3D Vertical-gate (VG) TFT NANDFlash Using Junction-Free Buried Channel BE-SONOS Device,” by H. T. Lueet al., published in the 2010 Symposium on VLSI: Tech. Dig. Of TechnicalPapers, pp. 131-132.

FIG. 1a shows three-dimensional vertical NAND strings 111 and 112 in theprior art. FIG. 1b shows basic circuit representation 140 of athree-dimensional vertical NAND string in the prior art. Specifically,vertical NAND string 111 and 112 of FIG. 1a and their circuitrepresentation 150 are each essentially a conventional horizontal NANDstring which—rather than each connecting 32 or more transistors inseries along the surface of a substrate—is rotated 90 degrees, so as tobe perpendicular to the substrate. Vertical NAND strings 111 and 112 areserially-connected thin-film transistors (TFTs) in a stringconfiguration that rises like a skyscraper from the substrate, with eachTFT having a storage element and a control gate provided by one of theword line conductors in an adjacent stack of word line conductors. Asshown in FIG. 1b , in the simplest implementation of a vertical NANDstring, TFTs 15 and 16 are the first and last memory transistors of NANDstring 150, controlled by separate word lines WL0 and WL31,respectively. Bit line select transistor 11, activated by signal BLS,and ground select transistor 12, activated by signal SS, serve toconnect an addressed TFT in vertical NAND string 150 to correspondingglobal bit line GBL at terminal 14 and global source line (ground) GSL,at terminal 13, during read, program, program-inhibit and eraseoperations. Reading or programming the content of any one TFT, (e.g.,TFT 17) requires activation of all 32 TFTs in vertical NAND string 150,which exposes each TFT to read-disturb and program-disturb conditions.Such conditions limit the number of TFTs that can be provided in avertical NAND string to no more than 64 or 128 TFTs. Furthermore, thepolysilicon thin films upon which a vertical NAND string is formed havemuch lower channel mobility—and therefore higher resistivity—thanconventional NAND strings formed in a single-crystal silicon substrate,thereby resulting in a low read current relative to the read current ofa conventional NAND string.

U.S. Patent Application Publication 2011/0298013 (“Hwang”), entitled“Vertical Structure Semiconductor Memory Devices And Methods OFManufacturing The Same,” discloses three-dimensional vertical NANDstrings. In its FIG. 4D, Hwang shows a block of three dimensionalvertical NAND strings addressed by wrap-around stacked word lines 150(reproduced herein as FIG. 1c ).

U.S. Pat. No. 5,768,192 to Eitan, entitled “Memory Cell utilizingasymmetrical charge trapping” filed Jul. 23 1996 and issued Jun. 16,1998 discloses NROM type memory transistor operation of the typeemployed in an embodiment of the current invention.

U.S. Pat. No. 8,026,521 to Zvi Or-Bach et al, entitled “SemiconductorDevice and Structure,” filed on Oct. 11, 2010 and issued on Sep. 27,2011 to Zvi-Or Bach et al discloses a first layer and a second layer oflayer-transferred mono-crystallized silicon in which the first andsecond layers include horizontally oriented transistors. In thatstructure, the second layer of horizontally oriented transistorsoverlays the first layer of horizontally oriented transistors, eachgroup of horizontally oriented transistors having side gates.

Transistors that have a conventional non-volatile memory transistorstructure but short retention times may be referred to as“quasi-volatile.” In this context, conventional non-volatile memorieshave data retention time exceeding tens of years. A planarquasi-volatile memory transistor on single crystal silicon substrate isdisclosed in the article “High-Endurance Ultra-Thin Tunnel Oxide inMonos Device Structure for Dynamic Memory Application”, by H. C. Wannand C. Hu, published in IEEE Electron Device letters, Vol. 16, No. 11,November 1995, pp 491-493. A quasi-volatile 3-D NOR array withquasi-volatile memory is disclosed in the U.S. Pat. No. 8,630,114 to H.T Lue, mentioned above.

The Article “A 768 Gb 3b/cell 3D-Floating-Gate NAND Flash Memory,” by T.Tanaka et al., published in the Digest of Technical Papers, the 2016IEEE International Solid-State Circuits Conference, pp. 142-144,discloses placing CMOS logic circuits underneath a 3-dimensional NANDmemory array.

SUMMARY

According to one embodiment of the present invention, a high-densitymemory structure, referred to as a three-dimensional vertical NOR Flashmemory string (“multi-gate vertical NOR string,” or simply “vertical NORstring”). The vertical NOR string includes a number of thin-filmtransistors (“TFTs”) connected in parallel, having a shared sourceregion and a shared drain region each extending generally in a verticaldirection. In addition, the vertical NOR string includes multiplehorizontal control gates each controlling a respective one of the TFTsin the vertical NOR string. As the TFTs in a vertical NOR string areconnected in parallel, a read current in a vertical NOR string isconducted over a much lesser resistance than the read current through aNAND string of a comparable number of TFTs. To read or program any oneof the TFTs in a vertical NOR string, only that TFT needs to beactivated, all other TFTs in the vertical NOR string can remainnon-conducting. Consequently, a vertical NOR string may include manymore TFTs (e.g., several hundreds or more), while allowing fastersensing and minimizing program-disturb or read-disturb conditions.

In one embodiment, the shared drain region of a vertical NOR string isconnected to a global bit line (“voltage V_(bl)”) and the shared sourceregion of the vertical NOR string is connected to a global source line(“voltage V_(ss)”). Alternatively, in a second embodiment, only theshared drain region is connected to a global bit line biased to a supplyvoltage, while the shared source region is pre-charged to a voltagedetermined by a quantity of charge in the shared source region. Toperform the pre-charge, one or more dedicated TFTs may be provided topre-charge the parasitic capacitance C of the shared source region.

According to one embodiment of the present invention, multi-gate NORflash thin-film transistor string arrays (“multi-gate NOR stringarrays”) are organized as arrays of vertical NOR strings each runningperpendicular to the surface of a silicon substrate. Each multi-gate NORstring array includes a number of vertical active columns arranged inrows, each row extending along a first horizontal direction, with eachactive column having two vertical heavily-doped polysilicon regions of afirst conductivity, which are separated by one or more verticalpolysilicon regions that are undoped or lightly doped to a secondconductivity. The heavily-doped regions each form a shared source ordrain region and, in conjunction with one or more stacks of horizontalconductors each extending orthogonally to the first horizontaldirection, the lightly-doped regions each form multiple channel regions.A charge-trapping material forms multiple storage elements, covering atleast the channel regions of TFTs in the active column. The horizontalconductive lines in each stack are electrically isolated from each otherand form control gates over the storage elements and the channel regionsof the active column. In this manner, the multi-gate NOR string arrayforms a three-dimensional array of storage TFTs.

In one embodiment, support circuitry is formed in a semiconductorsubstrate to support multiple multi-gate NOR string arrays formed abovethe support circuitry and the semiconductor substrate. The supportcircuitry may include address encoders, address decoders, senseamplifiers, input/output drivers, shift registers, latches, referencecells, power supply lines, bias and reference voltage generators,inverters, NAND, NOR, Exclusive-Or and other logic gates, other memoryelements, sequencers and state machines, among others. The multi-gateNOR string arrays may be organized as multiple blocks of circuits, witheach block having multiple multi-gate NOR string arrays.

According to embodiments of the present invention, variations inthreshold voltages of TFTs within a vertical NOR string may becompensated by providing one or more electrically programmable referencevertical NOR strings in the same or another multi-gate vertical NORstring array. Background leakage currents inherent to a vertical NORstring can be substantially neutralized during a read operation bycomparing the results of the TFT being read to that of a TFT that isconcurrently read on a programmable reference vertical NOR string. Insome embodiments, each TFT of a vertical NOR string is shaped so as toamplify the capacitive coupling between each control gate and itscorresponding channel region thereby to enhance tunneling from thechannel regions into the charge-trapping material (i.e., the storageelement) during programming, and to reduce the charge injection from thecontrol gate to the charge-trapping material during erasing. Thisfavorable capacitive coupling is particularly useful for storing morethan one bit in each TFT of a vertical NOR string. In anotherembodiment, the charge-trapping material of each TFT may have itsstructure modified to provide a high write/erase cycle endurance, albeitat a lower retention time that requires refreshing of the stored data.However, as the refreshing required of a vertical NOR string array isexpected to be much less frequently than in a conventional dynamicrandom-access memory (DRAM), the multi-gate NOR string arrays of thepresent invention may operate in some DRAM applications. Such use of thevertical NOR strings allows a substantially lower cost-per-bit figure ofmerit, as compared to conventional DRAMs, and a substantially lowerread-latency, as compared to conventional NAND string arrays.

In another embodiment the vertical NOR string can be programmed, erasedand read as NROM/Mirror-bit TFT string.

Organizing the TFTs as vertical NOR strings—rather than the prior artvertical NAND strings—results in (i) a reduced read-latency that canapproach that of a dynamic random access memory (DRAM) array, (ii)reduced sensitivities to read-disturb and program-disturb conditionsthat are associated with long NAND Flash strings, and (iii) reduced costper bit, as compared to a NAND Flash string.

According to an alternative embodiment of the present invention, eachactive column in the memory structure includes one or more vertical NORstrings, with each NOR string having thin-film storage transistorssharing a local source line and a local bit line, the local bit line isconnected by one segment of a segmented global bit line to a senseamplifier provided in the semiconductor substrate. To significantlyreduce the read sense latency, rather than a global bit line that spansa substantial distance (e.g., between a half to the complete length ofthe chip), multiple, shorter global bit line segments are provided. Eachsuch global segment connects one or more neighboring local bit linesthrough a segment connector to a segment sense amplifier provided in thesemiconductor substrate. In embodiments in which the local source linesare pre-charged to a virtual ground voltage (e.g., V_(ss)), theparasitic capacitance of the virtual ground is increased substantiallyby providing a short global source line segment connector which connectsa group of neighboring local source lines into one local source linesegment. The number of local source lines included in the segmentdetermines the combined parasitic capacitance (C).

The present invention is better understood upon consideration of thedetailed description below, in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a shows three-dimensional vertical NAND strings 111 and 112 in theprior art.

FIG. 1b shows basic circuit representation 140 of a three-dimensionalvertical NAND string in the prior art.

FIG. 1c shows a three-dimensional representation of a block ofthree-dimensional vertical NAND strings addressed by wrap-around stackedword lines 150.

FIG. 2 shows conceptualized memory structure 100, which illustrates a3-dimensional organization of memory cells; the memory cells areprovided in vertical NOR strings, with each vertical NOR string havingmemory cells each being controlled by one of a number of horizontal wordlines, according to one embodiment of the present invention.

FIG. 3a shows a basic circuit representation in a Z-Y plane of verticalNOR string 300 formed in an active column; vertical NOR string 300represents a three-dimensional arrangement of non-volatile storage TFTs,with each TFT sharing local source line (LSL) 355 and local bit line(LBL) 354, being accessed respectively by global bit line (GBL) 314 andglobal source line (GSL) 313 according to one embodiment of the currentinvention.

FIG. 3b shows a basic circuit representation in a Z-Y plane of verticalNOR string 305 formed in an active column; vertical NOR string 305represents a three-dimensional arrangement of non-volatile storage TFTs,including a dedicated pre-charge TFT 370 for setting a voltage(“V_(ss)”) on shared local source line 355, which has a parasiticcapacitance C, according to one embodiment of the present invention.

FIG. 3c shows a basic circuit representation of dynamic non-volatilestorage transistor 317 having one or more programmed threshold voltagesand connected to parasitic capacitor 360; capacitor 360 is pre-chargedto temporarily hold a virtual voltage V_(ss) on source terminal 355 soas to allow the threshold voltage of transistor 317 to be dynamicallydetected by the discharging of voltage V_(ss), when control gate 323 pis raised to a voltage that exceeds the threshold voltage.

FIG. 3d shows a variation of the vertical NOR memory array circuitarchitecture in the embodiment of FIG. 3a , in which global bit line(GBL) 314 is replaced by bit line segments MSBL₁, MSBL₂, . . . , eachconnecting multiple neighboring local vertical bit lines 374-1, 374-2, .. . ; the segments are in turn connected through segment-selectthin-film transistors 586-1, . . . , 586-n to regional bit line segmentsSGBL₁, SGBL₂, . . . that are each associated with multiple bit linesegments, and that are isolated by dielectric 393 from the senseamplifiers and other circuitry in silicon substrate 310 below them.

FIG. 3e shows a variation in the circuit architecture in the embodimentof FIG. 3d , in which global source-select line 313 accesses, throughsource-select transistor SLS₁, a group of neighboring vertical localsource lines 375-1, 375-2, . . . that are associated with source linesegment MSSL₁.

FIG. 3f shows a variation in circuit architecture in the embodiment ofFIG. 3e , in which global source line 313 is eliminated and replaced bylocal source line segment MSSL₁ connecting vertical local source lines375-1, 375-2, . . . , which are charged and held at virtual groundvoltage Vss through pre-charge transistors (e.g., pre-charge transistor370).

FIG. 3g shows a variation in circuit architecture in the embodiment ofFIG. 3f , in which regional bit line segments SGBL₁, SGBL₂, . . . , aremerged with bit line segments MSBL₁, MSBL₂, . . . , and are connectedthrough vias 322 to segment-select transistors 315-1, 315-2 . . . ,which are located in the substrate (thereby replacing segment-selectthin-film transistors 586-1, 586-2, . . . , of FIG. 3 d.

FIG. 3h shows a circuit architecture of the embodiment in FIG. 3g , inwhich two neighboring bit line segments MSBL₁, MSBL₂ have their localsource line segments MSSL₁, MSSL₂ connected from substrate 310 throughdedicated active vertical column 381 formed in the space labeled BL0between the two bit line segments.

FIG. 3i and FIG. 3i 1 together show a top X-Y plane view of theembodiment of FIG. 3h , in which each vertical local source line insource segment MSSL₁ is held at voltage Vss or Vbl supplied throughcolumn 381.

FIG. 4a is a cross section in a Z-Y plane showing side-by-side activecolumns 431 and 432, each of which may form a vertical NOR string thathas a basic circuit representation illustrated in either FIG. 3a or FIG.3b , according to one embodiment of the present invention.

FIG. 4a -1 is a top view of a vertical NOR string of FIG. 4a , in whichthe conductivity of the vertical local source line or drain line isenhanced by including metallic material 420(M) in the core of thepillars of the local source line or drain line.

FIG. 4b is a cross section in the Z-X plane showing active columns 430R,430L, 431R and 431L, charge-trapping layers 432 and 434, and word lines423 p-L and 423 p-R, according to one embodiment of the presentinvention.

FIG. 4c shows a basic circuit representation in the Z-X plane ofvertical NOR string pairs 491 and 492, according to one embodiment ofthe present invention.

FIG. 5a is a cross section in the Z-Y plane showing connections of avertical NOR string of active column 531 to global bit line 514-1(GBL₁), global source line 507 (GSL₁), and common body bias source 506(V_(bb)), according to one embodiment of the present invention.

FIG. 5b is a cross section in the Z-Y plane showing, according to oneembodiment of the present invention, connection of body region 556(providing the P⁻ channel material) to conductive pillar 591, which isformed in dielectric layer 592 out of P⁺ polysilicon, for example, toconductor 590 provided above active column 581 and running parallel tothe word lines; conductor 590 receives body bias voltage V_(bb) fromvoltage source 594 in substrate 505 through via 593 in an openingthrough dielectric isolation 509.

FIG. 6a is a cross section in the X-Y plane showing, according to oneembodiment of the present invention, TFT 685 (T_(L)) of vertical NORstring 451 a and TFT 684 (T_(R)) of vertical NOR string 451 b invertical NOR string pair 491, as discussed in conjunction with FIG. 4c ;in FIG. 6a , global bit line 614-1 accesses alternate ones of local bitlines LBL-1, and predetermined curvature 675 of transistor channel 656Lamplifies the capacitive coupling between each control gate and thecorresponding channel during programming.

FIG. 6b is a cross section in the X-Y plane showing, according to oneembodiment of the current invention, TFT 685(T_(L)) of vertical NORstring 451 a sharing an active region with TFT 684 (T_(R)) of verticalNOR string 451 b in vertical NOR string pair 491, as discussed inconjunction with FIG. 4c ; in FIG. 6b , global bit line 614-1 accessesalternate (odd) ones of local bit lines 654 (LBL-1), global bit line614-2 addresses alternate (even) ones of local bit lines 657-2 (LBL-2),local source lines LSL-1 and LSL-2 are pre-charged to provide virtualsupply voltage V_(ss).

FIG. 6c is a cross section in the X-Y plane showing, in accordance withone embodiment of the current invention, dedicated word line stacks 623p, each having word lines each surrounding (“wrapping around”) a TFT ofa vertical NOR string, and local vertical pillar bit line 654 (extendingalong the Z-direction) and local vertical pillar source line 655(extending along the Z-direction), which are accessed by globalhorizontal bit line 614 and global horizontal source line 615,respectively; in FIG. 6c , adjacent word line stacks 623 p are isolatedfrom each other by air gap 610 or another dielectric isolation.

FIG. 6d is a cross section in the X-Y plan showing, according to theembodiment of the present invention, staggered close-packing of verticalNOR strings, similar to those shown in FIG. 6c , sharing word-linestacks 623 p and with pre-charged parasitic capacitors 660 eachproviding a pre-charged virtual V_(ss) supply voltage.

FIG. 6e shows in the X-Y plane providing body bias voltage V_(bb) (e.g.,through conductors 690-1 and 690-2) that is shared between body regions656 (L+R) in adjacent rows of active columns, using the layout of theembodiment shown in FIG. 6 b.

FIG. 6f illustrates one implementation of global word lines forconnecting the local word lines on one plane (i.e., at one stair-casestep) in conjunction with the bit line segmentation scheme of thepresent invention.

FIG. 6g illustrates one implementation of a vertical NOR string memoryarray that avoids doubling of the silicon area taken up by word linestair-case steps when the number of layers of storage transistors aredoubled in the vertical direction, according to one embodiment of thepresent invention.

FIGS. 7a, 7b, 7c and 7d are cross sections of intermediate structuresformed in a fabrication process for a multi-gate NOR string array, inaccordance with one embodiment of the present invention.

FIG. 7d -1 shows in the X-Y plane the inclusion of conductive material720(M) at the core of vertical pillars of the local source line or localbit line.

FIG. 8a is a schematic representation of a read operation forembodiments where the local source line (LSL) of a vertical NOR stringis hard-wired; in FIG. 8a , “WLs” represents the voltage on the selectedword line, and all non-select word lines (“WL_(NS)”) in the vertical NORstring are set at 0V during the read operation.

FIG. 8b is a schematic representation of a read operation forembodiments where the local source line is floating at pre-chargevirtual voltage V_(ss); in FIG. 8b , “WL_(CHG)” represents the gatevoltage on the pre-charge transistor (e.g., pre-charge transistor 317 or370 in FIG. 3c ).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 shows conceptualized memory structure 100, which illustrates a3-dimensional organization of memory cells (or storage elements)provided in vertical NOR strings. In conceptualized memory structure100, each vertical NOR string includes memory cells that are eachcontrolled by a corresponding horizontal word line, according to oneembodiment of the present invention. In conceptualized memory structure100, each memory cell is formed in deposited thin films provided“vertically”, i.e., along a direction perpendicular to the surface ofsubstrate layer 101. Substrate layer 101 may be, for example, aconventional silicon wafer used for fabricating integrated circuits,familiar to those of ordinary skill in the art. In this detaileddescription, a Cartesian coordinate system (such as indicated in FIG. 2)is adopted solely for the purpose of facilitating discussion. Under thiscoordinate system, the surface of substrate layer 101 is considered aplane which is parallel to the X-Y plane. Thus, as used in thisdescription, the term “horizontal” refers to any direction parallel tothe X-Y plane, while “vertical” refers to the Z-direction.

In FIG. 2, each vertical column in the Z-direction represents storageelements or TFTs in a vertical NOR string (e.g., vertical NOR string121). The vertical NOR strings are arranged in a regular manner in rowseach extending along the X-direction. (Of course, the same arrangementmay also be seen alternatively as an arrangement of rows each extendingalong the Y-directions). The storage elements of a vertical NOR stringshare a vertical local source line and a vertical local bit line (notshown). A stack of horizontal word lines (e.g., WL 123) run along theY-direction, with each word line serving as control gates forcorresponding TFTs of vertical NOR strings located adjacent the wordline along the Y-direction. Global source lines (e.g., GSL 122) andglobal bit lines (e.g., GBL 124) are provided along the X-directiongenerally running either below the bottom of or on top of conceptualizedmemory structure 100. Alternatively, signal lines GSL 122 and GBL 124can both be routed below (or both be routed on top of) conceptualizedmemory structure 100, each of these signal lines may be selectivelyconnected by access transistors (not shown) to the local source linesand local bit lines of individual vertical NOR strings. Unlike avertical NAND string of the prior art, in a vertical NOR string of thepresent invention, writing or reading any one of its storage elementsdoes not involve activation of any other storage element in the verticalNOR string. As shown in FIG. 2, solely for illustrative purpose,conceptualized memory block 100 is a multi-gate vertical NOR stringarray consisting of a 4×5 arrangement of vertical NOR strings, with eachNOR string typically having 32 or more storage elements and accessselection transistors. As a conceptualized structure, memory block 100is merely an abstraction of certain salient characteristics of a memorystructure of the present invention. Although shown in FIG. 2 as a 4×5arrangement of vertical NOR strings, with each vertical NOR stringshaving a number of storage elements, a memory structure of the presentinvention may have any number of vertical NOR strings in each row alongeither of the X- and Y-directions, and any number of storage elements ineach vertical NOR string. For example, there may be thousands ofvertical NOR strings arrayed in rows along both the X- and Y-directions,with each NOR string having, for example, 2, 4, 8, 16, 32, 64, 128, ormore storage elements.

The number of storage elements in each vertical NOR string of FIG. 2(e.g., vertical NOR string 121) corresponds to the number of word lines(e.g., WL 123) providing control gates to the vertical NOR string. Theword lines are formed as narrow, long metallic strips each extendingalong the Y-direction. The word lines are stacked one on top of eachother, and electrically isolated from each other by dielectricinsulation layers there-between. The number of word lines in each stackmay be any number, but preferably an integer power of 2 (i.e., 2^(n),where n is an integer). The selection of a power of 2 for the number ofword lines follows a customary practice in conventional memory design.It is customary to access each addressable unit of memory by decoding abinary address. This custom is a matter of preference and need not befollowed. For example, within the scope of the present invention,conceptualized memory structure 100 may have M vertical NOR stringsalong each row in the X- and Y-directions, with M being a number that isnot necessarily 2^(n), for any integer n. In some embodiments to bedescribed below, two vertical NOR strings may share a vertical localsource line and a vertical local bit line, but their respective storageelements are controlled by two separate word line stacks. Thiseffectively doubles the storage density of the vertical NOR string.

As conceptualized memory structure 100 of FIG. 2 is provided merely toillustrate an organization of memory cells, it is not drawn to specificscale in any of the X-, Y-, and Z-directions.

FIG. 3a shows a basic circuit representation in a Z-Y plane of verticalNOR string 300 formed in an active column; vertical NOR string 300represents a three-dimensional arrangement of non-volatile storage TFTs,with each TFT sharing local source line 355 and local bit line 354,according to one embodiment of the current invention. In this detaileddescription, the term “active region,” “active column” or “active strip”refers to a region, column or strip of one or more semiconductormaterials on which an active device (e.g., a transistor or a diode) maybe formed. As shown in FIG. 3a , vertical NOR string 300 runs along theZ-direction, with TFTs 316 and 317 connected in parallel betweenvertical local source line 355 and vertical local drain or bit line 354.Bit line 354 and source line 355 are spaced apart, with the regionthere-between (i.e., body region 356) providing channel regions for theTFTs in the vertical NOR string. Storage elements are formed at theintersections between channel region 356 and each horizontal word line323 p, where p is the index of the word line in the word line stack; inthis example, p may take any value between 0 and 31. The word linesextend along the Y-direction. Local bit line 354 is connected throughbit line access select transistor 311 to horizontal global bit line(GBL) 314, which runs along the X-direction and connects local bit line354 to access bit line supply voltage V_(bl). Local source line 355 isconnected through horizontal global source line (GSL) 313 to sourcesupply voltage V_(ss). An optional source-select transistor (not shownin FIG. 3a ) can be provided to connect between local source line 355and GSL 313. The optional source-select transistor may be controlled bysource decoding circuitry which can be implemented either in thesubstrate (e.g., semiconductor substrate 101 of FIG. 2) or above thesubstrate and below memory structure 100, as is known to a personskilled in the art. Body region 356 of the active column may beconnected at terminal 331 to substrate bias voltage V_(bb). Substratebias voltage V_(bb) may be used, for example during an erase operation.The V_(bb) supply voltage can be applied to an entire multi-gatevertical NOR string array, or be applied selectively to one or more rowsof vertical NOR strings via a decoding mechanism. Lines connecting theV_(bb) supply voltage to body region 356 run preferably along thedirection of the word lines.

FIG. 3b shows a basic circuit representation in a Z-Y plane of verticalNOR string 305 formed in an active column; vertical NOR string 305represents a three-dimensional arrangement of non-volatile storage TFTs,including (optionally) dedicated pre-charge TFT 370 for momentarilysetting a voltage (“V_(ss)”) on shared local source line 355, which hasa parasitic capacitance C (represented by capacitor 360), according toone embodiment of the present invention. Unlike vertical NOR string 300of FIG. 3a , vertical NOR string 305 does not implement GSL 313,replacing it with pre-charge transistor 370 which pre-charges parasiticcapacitor 360, which temporarily holds a voltage of V_(ss) volts. Underthis pre-charging scheme, global source lines (e.g., global source lines313 of FIG. 3a ) and its decoding circuitry are rendered unnecessary,thereby simplifying both the manufacturing process as well as circuitlayout, and providing a very tight footprint for each vertical NORstring. FIG. 3c highlights the structure of non-volatile storage TFT317, which can also be used, in addition to its normal storage function,to perform the pre-charge function of dedicated pre-charge transistor370. A dynamic read operation for TFT 317 is described below inconjunction with sensing the correct one of several threshold voltagesthat is programmed into storage element 334 of TFT 317.

FIG. 4a is a cross section in a Z-Y plane showing side-by-side activecolumns 431 and 432, each of which may form a vertical NOR string thathas a basic circuit representation illustrated in either FIG. 3a or FIG.3b , according to one embodiment of the present invention. As shown inFIG. 4a , active columns 431 and 432 each include vertical N+ dopedlocal source region 455 and vertical N+ doped local drain or bit lineregion 454, separated by lightly P− doped or undoped channel region 456.P− doped channel region 456, N+ doped local source region 455 and N+doped local drain or bit line region 454 may be biased to body biasvoltage V_(bb), source supply voltage V_(ss), and bit line voltageV_(bl), respectively. In some embodiments of the current invention, useof body bias voltage V_(bb) is optional, such as when the active stripis sufficiently thin (e.g., 10 nanometers or less). For a sufficientlythin active strip, the active region is readily fully depleted underappropriate voltage on the control gate, such that voltage V_(bb) maynot provide a solid supply voltage to the channel regions of the TFTsalong the vertical NOR string. Isolation region 436, which electricallyinsulates active columns 431 and 432, may be either a dielectricinsulator or an air-gap. A vertical stack of word lines 423 p,respectively labeled WL₀-WL₃₁ (and optionally WL_(CHG)), providescontrol gates to the TFTs in the vertical NOR strings formed in activecolumns 431 and 432. Word line stack 423 p is typically formed as longnarrow metallic conductors (e.g., tungsten, a silicide or silicide) thatextend along the Y-direction, electrically isolated from each other bydielectric layers 426, each typically formed out of silicon oxide (e.g.,SiO₂) or an air gap. A non-volatile storage element may be formed at theintersection of each word line 423 p and each P− doped channel region456 by providing a charge-trapping material (not shown) between wordline 423 p and P− doped channel region 456. For example, FIG. 4aindicates by dashed boxes 416 the locations where nonvolatile storageelements (or storage transistors) T₀ to T₃₁ may be formed. Dashed box470 indicates where a dedicated pre-charge transistor may be formed,which, when momentarily switched on, allows charge to be transferredfrom common local bit line region 454 to common local source line region455 when all transistors T₀ to T₃₁ are in their off state.

FIG. 4b is a cross section in the Z-X plane showing active columns 430R,430L, 431R and 431L, charge-trapping layers 432 and 434, and word linestacks 423 p-L and 423 p-R, according to one embodiment of the presentinvention. Similar to FIG. 4a , each of vertical word line stacks 423p-L and 423 p-R in FIG. 4b denotes a stack of long narrow conductors,where p is an index labeling the word lines in stack (e.g., word linesWL₀ to WL₃₁). As shown in FIG. 4b , each word line serves as controlgates for the nonvolatile TFTs in the vertical NOR strings formed onadjacent active columns 430-L and 431-R on opposite sides of the wordline (within region 490). For example, in FIG. 4b , word line WL₃₁ inword line stack 423 p-R serves as control gates for both transistor 416Lon active column 430L and transistor 416R on active column 431R.Adjacent word line stacks (e.g., word lines stacks 423 p-L and 423 p-R)are separated by a distance 495, which is the width of a trench formedby etching through successive word line layers, as described below.Active columns 430R and 430L, and their respective charge-trappinglayers 432 and 434, are subsequently formed inside the trench etchedthrough the word line layers. Charge-trapping layer 434 is providedinterposed between word line stack 423 p-R and vertical active columns431R and 430L. As elaborated below, during programming of transistor416R, charge injected into charge-trapping layer 434 is trapped in theportion of charge-trapping layer 434 within dash box 480. The trappedcharge alters the threshold voltage of TFT 416R, which may be detectedby measuring a read current flowing between local source region 455 andlocal drain region 454 on active column 431R (these regions are shown,e.g., FIG. 4a in the orthogonal cross section of the active column). Insome embodiments, pre-charge word line 478 (i.e., WL_(CHG)) is providedas control gate of pre-charge TFT 470 that is used to charge parasiticcapacitance C of local source line 455 (see, capacitor 360 of FIG. 3band local source line 455 of FIG. 4a ) to a ground or source supplyvoltage V_(ss). For expediency, charge-trapping layer 434 also providesa storage element in pre-charge transistor 470, which however is notitself used as a memory transistor. Pre-charging may alternatively beperformed using any of memory transistors T₀ to T₃₁ formed on activecolumn 431R. One or more of these memory transistors, in addition totheir storage function, can perform the function of the pre-chargetransistor. To perform the pre-charge, the voltage on the word line orcontrol gate is temporarily raised to a few volts above its highestprogrammable threshold voltage, thereby allowing voltage V_(ss) appliedto local bit line 454 to be transferred to local source line 455 (FIG.4a ). Having memory transistors T₀ to T₃₁ perform the pre-chargefunction eliminates the need for separate dedicated pre-charge TFT 470.Care must be taken, however, to avoid unduly disturbing the thresholdvoltage of such memory TFT when it is performing its pre-chargingfunction.

Although active columns 430R and 430L are shown in FIG. 4b as twoseparate active columns separated by an air-gap or dielectric insulation433, the adjacent vertical N+ local source lines may be implemented by asingle shared vertical local source line. Likewise, the vertical N+local drain or bit lines may be implemented by a single shared verticallocal bit line. Such a configuration provides “vertical NOR stringpair”. In that configuration, active columns 430L and 430R may be seenas two branches (hence the “pair”) in one active column. The verticalNOR string pair provides double-density storage through charge-trappinglayers 432 and 434 interposed between active columns 430R and 430L andword lines stacks 423 p-L and 423 p-R on opposite sides. In fact, activecolumns 430R and 430L may be merged into one active string byeliminating the air gap or dielectric insulation 433, yet still achievethe pair of NOR TFT strings implemented at the two opposite faces of thesingle active column. Such a configuration achieves the samedouble-density storage, as the TFTs formed in the opposite faces of theactive columns are controlled by separate word line stacks and areformed out of separate charge-trapping layers 434 and 432. Maintainingseparate thin active columns 430R and 430L (i.e., instead of mergingthem into one active column) is advantageous because TFTs on each activecolumn are thinner than the merged column and can therefore more readilybe fully depleted under appropriate control gate voltage conditions,thereby substantially reducing source-drain subthreshold leakage currentbetween vertical source regions 455 and vertical drain regions 454 ofthe active columns (FIG. 4a ). Having ultra-thin (and therefore highlyresistive) active columns is possible for even very long vertical NORstrings (e.g., 128 TFTs or longer) because the TFTs in a vertical NORstring are connected in parallel and because only one of the many TFTsis switched on at any one time, in contrast with the high resistance ofa NAND TFT string where TFTs in the string are connected in series andmust therefore all be switched on to sense any one of TFTs in thestring. For example, in a 32-TFT vertical NOR string, to be able to readtransistor T₃₀ (FIG. 4a ), the channel length of channel region 456 mayspan just 20 nanometers, as compared to the corresponding channel lengthof a NAND string, which may be 32 times longer, or 640 nanometers.

FIG. 4c shows a basic circuit representation in the Z-X plane ofvertical NOR string pairs 491 and 492, according to one embodiment ofthe present invention. As shown in FIG. 4c , vertical NOR strings 451 band 452 a share a common word line stack 423 p-R, in the manner shownfor the vertical NOR strings of active strips 430L and 431R of FIG. 4b .For their respective commonly-connected local bit lines, vertical NORstring pairs 491 and 492 are served by global bit line 414-1 (GBL₁)through access select transistor 411 and global bit line 414-2 (GBL₂)through access select transistor 414, respectively. For their respectivecommonly-connected local source lines, vertical NOR string pairs 491 and492 are served by global source line 413-1 (GSL₁) and global source line413-2 (GSL₂), respectively (source line select access transistors can besimilarly provided and are not shown in FIG. 4c ). As shown in FIG. 4c ,vertical NOR string pair 491 includes vertical NOR strings 451 a and 451b that share local source line 455, local bit line 454, and optionalbody connection 456. Thus, vertical NOR string pair 491 represent thevertical NOR strings formed on active columns 430R and 430L of FIG. 4b .Word line stacks 423 p-L and 423 p-R (where, in this example, 31≥p≥0)provide control gates for vertical NOR string 451 a and vertical NORstring 451 b, respectively. The word lines to control gates in the stackare decoded by decoding circuitry formed in the substrate to ensure thatappropriate voltages are applied to the addressed TFT (i.e., theactivated word line) and to the unaddressed TFTs (i.e., all othernon-activated word lines in the string). FIG. 4c illustrates how storagetransistors 416L and 416R on active columns 430L and 431R of FIG. 4b areserved by the same word line stack 423 p-R. Thus, vertical NOR string451 b of vertical NOR string pair 491 and vertical NOR string 452 a ofvertical string pair 492 correspond to the adjacent vertical NOR stringsformed on active columns 430L and 431R of FIG. 4b . Storage transistorsof vertical NOR string 451 a (e.g., storage transistor 415R) are servedby word line stack 423 p-L.

In another embodiment, the hard-wired global source lines 413-1, 413-2of FIG. 4c are eliminated, to be substituted for by a parasiticcapacitance (e.g., the parasitic capacitance represented by capacitor460 of FIG. 4c or capacitor 360 of FIG. 3c ) between shared N+localsource line 455—which is common to both vertical NOR strings 451 a and451 b—and its numerous associated word lines 423 p-L and 423 p-R. In avertical stack of 32 TFTs, each of the 32 word lines contribute theirparasitic capacitance to provide total parasitic capacitance C, suchthat it is sufficiently large to temporarily hold the voltage suppliedby pre-charge TFT 470 to provide a virtual source voltage V_(ss) duringthe relatively short duration of read or programming operations. In thisembodiment, the virtual source voltage temporarily held on the parasiticcapacitor (C) is provided to local source line 455 from global bit lineGBL₁ through access transistor 411 and pre-charge transistor 470.Alternatively, dedicated pre-charge transistor 470 can be eliminated, ifone or more of the memory TFTs in the vertical NOR sting are used, inaddition to their storage function, to pre-charge local source line 455,by bringing its word line voltage momentarily higher than its highestprogrammed voltage. Using a storage TFT for this purpose, care must betaken, however, to avoid over-programming the storage TFT. Using thevirtual V_(ss) voltage provides the significant advantage of eliminatinghard-wired global source lines (e.g., GLS₁, GLS₂) and their associateddecoding circuitry and access transistors, thereby materiallysimplifying the process flow and design challenges and resulting in asignificant more compact vertical NOR string.

FIG. 5a is a cross section in the Z-Y plane showing connections ofvertical NOR string of active column 531 to global bit line 514-1(GBL₁), global source line 507 (GSL₁), and common body bias source 506(V_(bb)), according to one embodiment of the present invention. As shownin FIG. 5a , bit-line access select transistor 511 connects GBL₁ withlocal bit line 554, and buried contact 556 optionally connects a P− bodyregion on the active strip to body bias source 506 (V_(bb)) in thesubstrate. Bit-line access select transistor 511 is formed in FIG. 5aabove active column 531. However, alternatively, bit-line access selecttransistor 511 may be formed at the bottom of active column 531 or insubstrate 505 (not shown in FIG. 5a ). In FIG. 5a , bit-line accessselect transistor 511 can for example be formed in an isolated island ofan N+/P−/N+ doped polysilicon stack together with access select wordline 585. When a sufficiently large voltage is applied to select wordline 585, the P− channel is inverted, thereby connecting local bit line554 to GBL₁. Word line 585 runs along the same direction (i.e., theY-direction) as the word lines 523 p which serve as control gates to theTFTs of the vertical NOR string. Word line 585 may be formed separatelyfrom word lines 523 p. In one embodiment, GBL₁ runs horizontally alongthe X-direction (i.e., perpendicular to the directions of the wordlines), and bit-line access select transistor 511 provides access tolocal bit line 554, which is the local bit line of merely one of manyvertical NOR strings that are served by GBL₁. To increase read andprogram operation efficiency, in a multi-gate NOR string array,thousands of global bit lines may be used to access in parallel thelocal bit lines of thousands of vertical NOR strings that are accessedby word line 585. In FIG. 5a , local source line 555 is connectedthrough contact 557 to global source line 513-1 (GSL₁), which may bedecoded, for example by decoding circuitry in substrate 505.Alternatively, as described already, the global source line may beeliminated by providing a virtual source voltage V_(ss) on local bitline 555 and temporarily pre-charging the parasitic capacitor 560 (i.e.,parasitic capacitance C) of local source line 555 through TFT 570.

Support circuitry formed in substrate 505 may include address encoders,address decoders, sense amplifiers, input/output drivers, shiftregisters, latches, reference cells, power supply lines, bias andreference voltage generators, inverters, NAND, NOR, Exclusive-Or andother logic gates, other memory elements, sequencers and state machines,among others. The multi-gate NOR string arrays may be organized asmultiple blocks of circuits, with each block having multiple multi-gateNOR string arrays.

FIG. 6a is a cross section in the X-Y plane, showing TFT 685 (T_(L)) ofvertical NOR string 451 a and TFT 684 (T_(R)) of vertical NOR string 451b in vertical NOR string pair 491, as discussed above in conjunctionwith FIG. 4c . As shown in FIG. 6, TFTs 684 and 685 share N+ localsource region 655 and N+ local drain or bit line region 654, bothregions extending in long narrow pillars along the Z-direction. (N+local source region 655 corresponds to local source line 455 of FIG. 4a, N+ local drain region 654 corresponds to local bit line 454 of FIG. 4a). In this embodiment, P− doped channel regions 656L and 656R form apair of active strings between local source pillar 655 and local drainpillar 654 and extend along the Z-direction, isolated from each other byisolation region 640. Charge-trapping layer 634 is formed between wordlines 623 p-L (WL₃₁₋₀) and 623 p-R (WL₃₁₋₁) and the outside of channelregions 656L and 656R respectively. Charge trapping layer 634 may be atransistor gate dielectric material consisting of, for example, a thinfilm of tunnel dielectric (e.g., silicon dioxide), followed by a thinlayer of charge trapping material such as silicon nitride or conductivenanodots embedded in a non-conducting dielectric material, or isolatedfloating gates, and is capped by a layer of blocking dielectric such asONO (an oxide-nitride-oxide triple-layer) or a high dielectric constantfilm such as aluminum oxide or hafnium oxide or some combination of suchdielectrics. Source-drain conduction is controlled by word lines 623 p-Land 623 p-R, respectively, forming control gates on the outside ofcharge-trapping layer 634. When programming or reading TFT 684 (T_(R)),TFT 685 (T_(L)) is turned off by maintaining an appropriate inhibitvoltage at word line 623 p-L. Similarly, when programming or reading TFT685 (T_(L)), TFT 684 (T_(R)) is turned off by maintaining an appropriateinhibit voltage at word line 623 p-R.

In the embodiment shown in FIG. 6a , word lines 623 p-L and 623 p-R arecontoured to enhance tunneling efficiency into the TFTs 684 and 685during programming, while reducing reverse-tunneling efficiency duringerasing. Specifically, as is known to a person skilled in the art,curvature 675 of channel region 656R amplifies the electric field at theinterface between the active channel polysilicon and the tunnelingdielectric during programming, while reducing the electric field at theinterface between the word line and the blocking dielectric duringerasing. This feature is particularly helpful when storing more than onebit per TFT transistor in a multi-level cell (MLC) configuration. Usingthis technique, 2, 3, or 4 bits or more may be stored in each TFT. Infact, TFTs 684 and 685 may be used as analog storage TFTs with acontinuum of stored states. Following a programming sequence (to bediscussed below), electrons are trapped in charge-trapping layer 634, asindicated schematically by dashed lines 680. In FIG. 6a , global bitlines 614-1 and 614-2 run perpendicularly to word lines 623 p-R and 623p-L and are provided either above or underneath the vertical NORstrings, corresponding to bit lines 414-1 and 414-2 respectively of FIG.4c . As discussed above in conjunction with FIG. 2, the word lines mayspan the entire length of memory block 100 along the X-direction, whilethe global bit lines span the width of memory block 100 along theY-direction. Of importance, in FIG. 6a , word line 623 p-R is shared byTFTs 684 and 683 of two vertical NOR strings on opposite sides of wordline 623 p-R. Accordingly, to allow TFTs 684 and 683 to be read orprogrammed independently, global bit line 614-1 (GBL₁) contacts localdrain or bit line region 657-1 (“odd addresses”), while global bit line614-2 (GBL₂) contacts local drain or bit line region 657-2 (“evenaddresses”). To achieve this effect, contacts along global bit lines614-1 and 614-2 are staggered, with each global bit line contactingevery other one of the vertical NOR string pair along the X-directionrow.

In like manner, global source lines (not shown in FIG. 6a ), which maybe located either at the bottom or above the multi-gate NOR stringarray, may run parallel to the global bit lines and may contact thelocal source lines of vertical NOR string pairs according to even or oddaddresses. Alternatively, where pre-charging of the parasitic capacitor(i.e., capacitor 660) temporarily to virtual source voltage V_(ss) isused, the global source lines need not be provided, thereby simplifyingthe decoding scheme as well as the process complexity.

FIG. 6a shows only one of several possible embodiments by which verticalNOR string pairs may be provided with stacked word lines. For example,curvature 675 in channel region 656R can be further accentuated.Conversely such curvature can be altogether eliminated (i.e.straightened out) as shown in the embodiment of FIG. 6b . In theembodiment of FIG. 6b isolation spacing 640 of FIG. 6a may be reduced oraltogether eliminated by merging channel regions 656L and 656R into asingle region 656(L+R), achieving greater area efficiency withoutsacrificing the dual-channel configuration: for example TFTs 685 (T_(L))and 684 (T_(R)) reside on opposite faces of the same active strip. Inthe embodiments of FIGS. 6a, 6b , vertical NOR strings sharing a wordline may be laid out in a staggered pattern relative to each other (notshown), such that they may be brought closer to each other, therebyreducing the effective footprint of each vertical NOR string. AlthoughFIGS. 6a and 6b show direct connection via a contact between global bitline 614-1 and N+ doped local drain bit line pillar 654 (LBL-1), suchconnection can also be accomplished using a bit-line access selectiontransistor (e.g., bit line access select transistor 511 of FIG. 5a , notshown in already crowded FIGS. 6a and 6b ).

In the embodiments of FIGS. 6a and 6b , dielectric isolation between N+doped local drain region 654 and its adjacent local N+ doped sourceregion 658 (corresponding to isolation region 436 of FIG. 4a ) can beestablished by, for example, defining the separation 676 between wordlines 623 p-R and 623 p-L to be less than the thicknesses of twoback-to-back charge-trapping layers, so that the charge-trapping layersare merged together during their deposition. The resulting merging ofthe deposited charge-trapping layers creates the desired dielectricisolation. Alternatively, isolation between adjacent active strings canbe achieved by using a high aspect-ratio etch of N+ polysilicon tocreate gap 676 (air gap or dielectric filled) isolating N+ pillar 658 ofone string from N+ pillar 654 of the adjacent string (i.e., creating gap436 shown in FIG. 4a ).

Contrasting between the prior art vertical NAND strings and the verticalNOR strings of the current invention, although both types of devicesemploy thin-film transistors with similar word line stacks as controlgates, their transistor orientations are different: in the prior artNAND string, each vertical active strip may have 32, 48 or more TFTsconnected in series. In contrast, each active column forming thevertical NOR strings of the present invention the vertical column mayhave one or two sets of 32, 48 or more TFTs connected in parallel. Inthe prior art NAND strings, the word lines in some embodiments typicallywrap around the active strip. In some embodiments of the vertical NORstring of the present invention separate designated left and right wordlines are employed for each active strip, thereby to achieve a doubling(i.e. a pair) storage density for each global bit line, as illustratedin FIGS. 4c, 6a and 6b . The vertical NOR strings of the presentinvention do not suffer from program-disturb or read-disturbdegradation, nor do they suffer from the slow latency of the prior artNAND strings. Thus, a much larger number of TFTs may be provided in avertical NOR string than in a vertical NAND strings. Vertical NORstrings, however, may be more susceptible to subthreshold or otherleakage between the long vertical source and drain diffusions (e.g.,local source region 455 and local drain region 454, respectively,illustrated in FIG. 4a ).

Two additional embodiments of the vertical NOR string of this inventionare shown in FIG. 6c and FIG. 6d . In these embodiments, all word linesin each word-line stack wrap around the vertical active strip.

In FIG. 6c , a vertical NOR string is formed inside the voids that areformed by etching through a stack of metal word lines and the dielectricisolation layers between the word lines. The manufacturing process flowis similar to that of the prior art vertical NAND strings, except thatthe transistors in a vertical NOR string are provided parallel to eachother, rather than serially in a vertical NAND string. Formation oftransistors in a vertical NOR string is facilitated by the N+ dopedvertical pillars extending to the entire depth of the void, providingshared local source line 655 (LSL) and shared local bit line (drain) 654(LBL) for all the TFTs along the vertical NOR string, with undoped orlightly doped channel region 656 adjacent to both. Charge storageelement 634 is positioned between channel 656 and word line stack 623 p,thus forming a stack of 2, 4, 8, . . . 32, 64 or more TFTs (e.g., device685 (T₁₀)) along the vertical active strip. In the embodiment of FIG. 6c, the word line stacks run in the Y-direction, with individualhorizontal strips 623 p (WL₃₁₋₀), 623 p (WL₃₁₋₁) being separated fromeach other by air gap or dielectric isolation 610. Global bit lines 614(GBL) and global source lines 615 (GSL) run horizontally in rows alongthe X-direction, perpendicular to the word lines. Each global bit line614 accesses local bit line pillars 654 (LBL) along the row of verticalstrips through access select transistors (511 in FIG. 5a , not shownhere) that can be positioned either below the memory array or above it.Similarly, each global source line 615 accesses the local source linepillars along the row. While the structures shown in FIGS. 6a and 6b areable to fit a pair of vertical NOR strings in roughly the same areataken up by a single vertical NOR string in the embodiment of FIG. 6c ,each TFT in each vertical NOR string shown in FIG. 6c has two parallelconduction channels (i.e., channel regions 656 a and 656 b), andtherefore may store more charge and increase or double the read current,thereby enabling storing more bits in each TFT.

FIG. 6d shows a more compact vertical NOR string with wrap-around wordlines, according to one embodiment of the present invention. As shown inFIG. 6d , vertical NOR strings are staggered as to be closer together,so that word line stack 623 p (WL₃₁₋₀) can be shared by more verticalNOR strings. The staggered configuration is enabled by using theparasitic capacitor (i.e., parasitic capacitors 660) of local sourceline pillar 655 (LSL). By pre-charging capacitors 660 to temporarilyhold virtual voltage V_(ss) during read and program operations, asdescribed below, the need for hard-wired global source lines (e.g., GSL615 in FIG. 6c ) is obviated. Although the vertical NOR strings of FIGS.6c and 6d may not by themselves offer significant areal efficiencies, ascompared to prior art vertical NAND strings (e.g., the NAND strings ofFIG. 1c ), such vertical NOR strings achieve much greater string lengthsthan vertical NAND strings. For example, while vertical NOR strings ofthe present invention may well support strings of length 128 to 512 ormore TFTs in each stack, such string lengths are simply not practicalfor a vertical NAND string, given the serious limitations attendant withseries-connected TFT strings.

Alternative Embodiments with Long Global Bit Lines that are Partitionedinto Short, Segmented Bit Lines to Facilitate Fast Access to SenseAmplifiers

The inventor notes that, with sense amplifiers and other supportcircuits provided in the semiconductor substrate, routing global bitlines using global interconnect conductors provided above or below amemory array to connect to vertical local bit lines (e.g., global bitline GBL1 connecting to the vertical local bit line 554 of FIG. 5a )results in large RC delays because of the substantial length of thewiring involved. Furthermore, it is highly desirable to use the area ofthe silicon substrate underneath the memory arrays (as opposed to takingup precious silicon area beside the arrays) to form the numerous supportcircuitry, such as sense amplifiers, decoders, voltage sources and othercircuits necessary for memory operations.

According to one embodiment of the present invention, a conductor thatotherwise would be used as a global bit line may be segmented into amultiplicity of relatively short line segments (e.g., each line segmentmay have a length that is 1/100 or less of the global bit line). Eachline segment provides a horizontal line connector for connecting a groupof neighboring vertical local bit lines. The bit line segment may residepreferably between, and dielectrically isolated from, the substrate andthe memory arrays. The bit line segment facilitates connections betweenthe neighboring vertical local bit lines in the group and dedicatedsense amplifiers and other support circuits formed in the semiconductorsubstrate underneath the array of vertical NOR strings. In this detaileddescription, the term “bit line segment” may refer to the collection oflocal bit lines connected by a line connector.

Similarly, a conductor that otherwise would be used as a global sourceline may also be segmented into a multiplicity of relatively short linesegments each providing a horizontal line connector for connecting agroup of neighboring local vertical source lines. The line connector andits associated local vertical source lines form a common source linewhose parasitic capacitance is multiple times the parasitic capacitanceof just one local vertical source line. The common source line connectormay be connected by a segment-select transistor to a global source line,preferably at the top of the array. In this detailed description, theterm “source line segment” may refer to the collection of local sourcelines connected by a line connector. Where the source line segment maybe further divided into smaller groups of connected local source lines,each such smaller group may be referred to as a “source linesub-segment.”

In another alternative embodiment of the present invention, globalsource lines running on top of, or below the memory stacks are notprovided, but each source line segment and its associated group ofneighboring local vertical source lines is operated as a local commonsource region. In that configuration, one or more of pre-chargetransistors are provided in each active column connected to the sourceline segment to transfer a virtual ground voltage (V_(ss)) from thesubstrate. In a 64-layer vertical NOR memory array, each local sourceline may have a parasitic capacitance that is about 1 femtofarad (i.e.,1.0×10⁻¹⁵ farads), which provides in some instances too small a chargeto maintain a virtual ground voltage (V_(ss)) during a charge-sharingread operation. By combining the capacitances of a group of, say 64local source lines, their combined pre-charged capacitance C isincreased to approximately 64 femtofarads, which would be more thanadequate for the charge-sharing read operation.

FIGS. 3d, 3e, 3f and 3g show embodiments of the present invention thatachieve fast read access and utilize the silicon substrate underneaththe array to form support circuitry, such as sense amplifiers, decoders,registers, and voltage sources. As shown in FIG. 3d , vertical NORstring 380 represents a three-dimensional arrangement of non-volatilestorage TFTs, with each TFT sharing local source line 375 and local bitline 374, according to one embodiment of the present invention. Localbit line 374 and local source line 375 are spaced apart by body region356, which provides channel regions for the TFTs in vertical NOR string380. Storage elements are formed at the intersections between channelregion 356 and each horizontal word line 323 p, where p is the index ofthe word line in the word line stack; in this example, p may take anyvalue between 0 and 31. The word lines extend along the Y-direction. Inthis embodiment, source line supply voltage Vss is provided to localvertical source line 375 through source select transistor (SLS) 371 fromsubstrate 310 through global source line (GSL₁) 313 shown running on topof the vertical column. Note that body region 356, providing thetransistor channels of the active column, may be connected at terminal331 to substrate bias voltage V_(bb). However, electrically connectingthe P− doped channel 556 can also be achieved from the top of thevertical NOR string (see below the discussion relating to FIG. 5b ).

In FIG. 3d , neighboring active columns (e.g., the active column ofvertical NOR string 380) are grouped, with the local bit lines of eachgroup of active columns being connected to an associated bit linesegment (e.g., bit line segments MSBL₁ and MSBL₂) provided beneath thememory array. Bit line segment MSBL₁ provides a low-resistivityconnector 373, which may be implemented by, for example, a narrow stripof N+ doped polysilicon, a silicide or a refractory metal. The group ofneighboring local vertical bit lines 374-1, 374-2, . . . 374-n connectedby horizontal bit line segment MSBL₁ may be provided lengthwise alongthe X-direction, orthogonally to word lines WL₀ to WL₃₁. Bit linesegments MSBL₁, MSBL₂, . . . are formed on dielectric insulator 392 andmay be relatively short, such as encompassing from 1 (i.e., nosegmentation) to 16, 64, 256, 512 or more vertical local bit lines. Eachbit line segment can be connected through a segment-select transistor(e.g., segment-select transistors 586-1, . . . , 586-n, which may beimplemented as thin-film transistors) to longer horizontal conductorsforming regional bit line segments SGBL₁, SGBL₂ that include multipleMSBL₁-type bit line segments. Horizontal regional bit line segment SGBL₁may be formed on an insulating layer 393 above substrate 310, to allowlogic elements such as sense amplifiers to be formed in the substrateimmediately underneath the regional bit line segment. Preferably theregional segment is sufficiently long to allow sense amplifiers,decoders, registers, voltage sources and other circuitry formed in thesubstrate to physically fit underneath the regional bit line segment.

In a double-density configuration, such as shown in FIG. 6e , each wordline services both active columns on both sides of the word line. Inthat configuration, two adjacent local bit lines on opposite sides ofthe word line are associated respectively with bit line segmentsMSBL₁(L) and MSBL₁(R) and their respective segment sense amplifiers anddecoders, which are closely spaced apart from and run parallel to eachother. This spacing is also the spacing along the Y-direction betweenadjacent vertical active columns in the memory array. It may not bepossible to provide a dedicated sense amplifier and other supportingcircuits for each of the bit line segments laid out along theY-direction. In such an arrangement, each sense amplifier may serve 1,2, 4, 8 or more adjacent bit line segments through a segment-selectdecoder in the substrate. In the X-direction, a 1-terabit 3-dimensionalvertical NOR flash memory chip may have hundreds of regional bit linesegments, rather than a long global bit line, thereby significantlyreducing the bit-line RC delay.

FIG. 3e shows a variation of the circuit architecture in the embodimentof FIG. 3d , in which groups of neighboring vertical local source lines375-1, 375-2, . . . are connected by source line segments MSSL₁, MSSL₂,. . . running along the same X-direction as the bit-line segments. Thisgrouping of local source lines connected by source line segments reducesthe number of source line select transistors SLS₁, SLS2, . . . needed toprovide source voltage V_(ss) to each of the vertical NOR stringsassociated with the source line segment. Furthermore, as previouslynoted, connecting a group of vertical local source lines by a sourceline segment contributes directly to increasing the cumulative parasiticcapacitance (C). The vertical local source lines connected by ahorizontal source line segment are also closely associated with thevertical local bit lines connected by the corresponding horizontal bitline segment. However, the number of vertical local bit-lines associatedwith a bit line segment need not be the same as the number of verticallocal source lines associated with a source line segment. As a result, abit line segment may be associated, for example, with multiple sourceline segments. For example, bit line segment MSBL₁ may be associatedwith 256 local vertical bit lines 374-1, 374-2, . . . , which may beassociated with eight source line segments, each of which may only beassociated with 32 local source lines 375-1, 375-2, . . . . Each sourceline segment can have its voltage V_(ss) separately imparted to itthrough its dedicated source-line select transistor (e.g., source-lineselect transistor SLS₁).

FIG. 3f shows a variation of the circuit architecture of the embodimentin FIG. 3e , in which neither global source line (e.g., global sourceline 313) nor source line-select transistor (e.g., source-selecttransistor SLS₁) is provided. In FIG. 3f , the local vertical sourcelines associated with each source line segment are pre-charged to sourcevoltage V_(ss) through a pre-charge transistor (e.g., pre-chargetransistor 370) whose word line W_(CHG) is turned on with a voltagepulse sufficient to transfer voltage V_(bl) supplied from the circuitryin substrate 310 through the associated local vertical bit linesassociated with the source line segment. The number of local verticalbit lines associated with the source line segment is an optimizationbetween maximizing the parasitic capacitance (C) of the source linesegment to hold the virtual ground voltage V_(ss) during a read of thecell, balanced by the need to keep the background leakage currentattendant to all the “off” transistors in the vertical NOR stringsassociated with the source line segment sufficiently low, so as not tointerfere with reading the accessed storage transistor within the sourceline segment. Within a bit line segment, any unselected source linesub-segment can be pre-charges to have its V_(ss) voltage equalized withits associated bit line segment voltage V_(bl) to eliminate itsbackground leakage current.

FIG. 3g is a variation of the circuit architecture in the embodiment ofFIG. 3e . In FIG. 3g , the connectivity between the memory array and thesubstrate is further simplified by merging regional bit line segmentsSGBL₁, SGBL2, . . . with their respective local bit line segments MSBL₁,MSBL₂, . . . , and having each bit line segment connecting throughrespective vias or conductors (e.g., buried contacts) to segment-selecttransistors 315-1, 315-2, . . . in the substrate underneath the bit linesegments. In this configuration, rather than providing thin-filmsegment-select transistors above the silicon substrate (e.g.,segment-select transistors 586-1, . . . , 586-n of FIG. 3f ), thesegment-select transistors are provided by high-efficiency transistorsin single-crystal substrate 310. This configuration provides robustaccess to the sense amplifiers, decoders, registers, voltage sources andother circuitry associated with the bit line segment. By eliminatingglobal source line select transistors SLS₁, SLS₂, . . . , made possibleby the pre-charge path, and by eliminating segment-select thin-filmtransistors 586-1, . . . , 586-n (or select transistors built withcostly selective epitaxy silicon, as is commonly done in conventional 3DNAND arrays), made possible by having each bit line segment positionedclose to its segment circuitry in the substrate, materially simplifiesthe process integration flow.

FIGS. 3h, 3i and 3i 1 show another embodiment similar to the embodimentof FIG. 3g . (As FIGS. 3i and 3i 1 are portions of one figure that isdivided in compliance with U.S. Patent and Trademark Office draftingrules, and are not intended to be viewed independently, a referencehereinafter to FIG. 3i refers to both FIGS. 3i and 3i 1). In FIGS. 3hand 3i , the voltage on source line segment connectors MSSL₁ and MSSL₂,and hence also the voltage on local vertical source lines 375(LSL)within each source line segment, is supplied from substrate 310 throughan active column 381 (“charging column”) that mimics in construction anyof the storage active columns (e.g., active column 380) of the memoryarray but is however not used for memory storage. In other words,charging column 381 is dedicated to charging the local source lines insource line segments MSSL₁ and MSSL₂. (In other embodiments, eachcharging column may supply only a single source line segment.) As shownin FIG. 3h , charging column 381 may be formed, for example, in theopening BLO between neighboring bit line segments SEG₁ and SEG₂.Throughout a read operation (and optionally, any programming,program-inhibit, or erase operation), charging column 381 delivers andholds a required voltage on the vertical local source lines in sourceline segments MSSL₁ and MSSL₂. (Source line segments MSSL₁ and MSSL₂ areboth served by charging column 381.) In this regard, charging column 381obviates the need for global source line GSL1 313 of FIG. 3e , forexample, and eliminates the need for the associated source linesegment-select transistor SLS₁. It also eliminates, for example, theneed for pre-charge transistors 370—which requires extra word line planeWL_(chg)—in the memory stack, such as shown for the embodiment of FIG. 3g.

In the segmentation structure of FIGS. 3h and 3i , in a read operationof any storage transistor on any of the memory planes, the sourcevoltage on each local source line of source line segments MSSL₁ andMSSL₂ are imposed at V_(ss) (e.g., 0 volts) through connection VSL fromvertical source line 375(LSL) of charging column 381. Voltage V_(ss) isdelivered from substrate 310 through a decoded select transistor (shownin FIG. 3h as 315X) in silicon substrate 310, bit-line mini-segmentSSVss, vertical local bit line 374(LBL), pass transistor 371 andvertical local source line 375(LSL). (Pass transistor 371 is activatedand held in the conducting or “on” state by word line WL₃₁ throughoutthe read operation.) Source voltage to be imposed on source linesegments MSSL₁ and MSSL₂ during any programming, program-inhibit orerase operation may be similarly provided. Select transistor 315X insilicon substrate 310 may be a high voltage transistor that is able towithstand a high voltage imposed on local bit line 374 (LBL) during anerase operation.

FIG. 3i shows in greater detail a top X-Y plane view of the embodimentof FIG. 3h , in which each vertical local source line in source segmentMSSL₁ is held at voltage V_(ss) or V_(bl) supplied through column 381.In FIG. 3i , the memory array has a layout similar to that shown in theembodiment of FIG. 6b . As shown in FIG. 3i , between bit line segmentsSEG₁ and SEG₂ is provided an array of charging columns, with each rowextending along the X-direction having two charging columns and apredetermined number (e.g. 2048) of such rows laid out in along theY-direction. This array of charging columns is provided between the twodiscontinuities or openings in the bit lines (labeled in FIG. 3i as“BLO”.) In one row of the active columns, between the two dash lines, asource line connector extending along the X-direction connects the rightcharging column to the local source lines in source line segment MSSL₁(i.e., every other active column along the upper dash line) in bit linesegment SEG₁. The same right charging column is connected to the localsource lines of the active columns of source line segment MSSL₂ in bitline segment SEG₂. The source voltage is provided from the siliconsubstrate to a bit line connector to the local bit line of the rightactive column. The word lines labeled “WL₃₁” activate a pass transistorin the charging column to transfer the source voltage to the localsource line labeled VSL, which provides the source voltage to the localsource lines of source line segments MSSL₁ and MSSL₂. (This circuitconfiguration is shown in the circuit of FIG. 3h .) The left chargingcolumn in this row of charging columns between the dash lines isconnected to another pair of source line segments along the lower dashline in similar manner.

In a 3-dimensional vertical NOR string memory array having multipleword-line planes, the local word lines for all planes in a stack may bearranged in staircase steps WL_(STC) at the edge of the array (see,e.g., FIG. 3i and FIG. 6g ). One or more dedicated global word lines(labeled, for example, “GWL_(chg)” in FIG. 3i ) may be required for eachmemory plane to activate a charging column (e.g., charging column 381)for each pair of neighboring bit line segments (e.g., bit line segmentsSEG₁ and SEG₂ in FIG. 3h ). As shown in the example of FIG. 3i (see theinsert), the global word lines labeled GWL_(chg) are all connected tolocal word line WL₃₁ corresponding to active column 381 and skip overall other word lines in bit line segments SEG₁ and SEG₂. In contrast,each global word line for the storage transistors of the memory array(e.g., GWL) is hard wire-connected to the numerous local word linesassociated with bit line segments SEG₁ and SEG₂, while skipping over theword lines of charging column 381. The global word lines of chargingcolumn 381 (all labeled “GWL_(chg)” in the insert of FIG. 3i ) ondifferent memory planes can be shorted together at the peripherycircuitry (not shown), thereby activating any (or all) of the passtransistors of charging column 381 associated with word lines WL₀-WL₃₁.In one embodiment, the pass transistors of all charging columns in ablock of connected source line segments may be activated together whenthe chip is powered up; however, any source line segment or source linesegment pair within the block can be unselected by having itscorresponding charging column isolated from the silicon substrate byswitching off its associated segment-select transistor (e.g.,segment-select transistor 315X).

The embodiment of FIGS. 3h and 3i eliminates the need for a pre-chargesequence of the floating source, such as performed in the embodiment ofFIG. 3g . Eliminating the pre-charge sequence speeds up a read operationbecause the source voltage can be set and then held steady at voltageV_(ss) before the start of the read operation, thus eliminating theoverhead time required for the floating source pre-charge pulse.Furthermore, as charging column 381 holds the local source lines ofsource line segment MSSL₁ at voltage Vss throughout the read operation(i.e. not just a momentary pre-charge pulse), the steady currentprovided through connection VSL compensates for any source-drain leakagewhich, if excessive, could compromise the read sensing of the addressedstorage transistor.

To summarize, charging column 381 serves as a local vertical connectorfor transferring voltages V_(ss) or V_(bl) from the silicon substrate tothe local source lines in the vertical NOR memory strings. Any voltageV_(ss) or V_(bl) on the vertical local source line of a charging columncan be transferred to its associated local bit line through a passtransistor (e.g., pass transistor 371), although the local bit line mayalso be directly charged from bit line connector MSBL1, which may beconnected to voltage sources in the silicon substrate throughsegment-select decoders 315-1.

In a three-dimensional vertical NOR memory stack with 64 or 128 memoryplanes, the height of the stack, which is also the length of chargingcolumn 381, can exceed 5 microns, which is a rather long distance forvertical local source line 375(LSL) or local bit line 374 (LBL) ofcharging column 381 (FIG. 3h ). The electrical resistance (R; in ohms)of the corresponding N+ doped polysilicon pillars 455 and 454 (see, FIG.4a ; also shown as 655(N+) LSL-1 and 654 (N+) LBL-1 in FIG. 6e andsometimes referred to as pylons) may become excessive, introducing an RCdelay that adversely impacts the read path primarily. The pillar'sresistance R can be reduced by an order of magnitude or more byproviding a low-resistivity metallic material in the core of the pillar.For example, in the detailed description below, FIG. 4a -1 showsmetallic core 420 (M) and FIG. 7d -1 shows metallic core 720 (M).

FIG. 5b is a cross section in the Z-Y plane showing, according to oneembodiment of the present invention, connection of body region 556(providing the P⁻ channel material) by conductive pillar 591, which isformed in dielectric layer 592 out of P⁺ polysilicon, for example, to aconductor 590 provided above active column 581 and running in oneconfiguration parallel to the word lines. Conductor 590 may also beformed out of heavily doped polysilicon, or a silicide or metallicconductor. In this arrangement, body bias voltage (V_(bb)) 594 can beprovided to conductor 590 from substrate 505 through via 593 in anopening in dielectric isolation 509, to facilitate block eraseoperations.

FIG. 6e illustrates providing the body bias voltage through conductors690-1 and 690-2 (“body bias conductors”). The body bias voltage isshared between body regions in adjacent rows of active columns, usingthe layout of the embodiment shown in FIG. 6b . In this configuration,word line 592 (i.e., word line 623 p-L) runs coincidentally with bodybias conductor 690-1. The block size of an erase operation is limited tothe active columns on the left and the active columns on the right ofeach body bias conductor (e.g., conductor 690-1). Larger erase blockscan be configured for example by having a cluster of body biasconductors tied together to match the number of word lines addressing abit line segment. A decoder in the substrate provides the appropriatebody bias voltage (e.g., the erase voltage) to one or more selectederase blocks.

Referring back to FIG. 5b , after the active columns (e.g., activecolumn 581) are formed, dielectric layer 592 is formed over the activecolumns. Thereafter, via holes are anisotropically etched from the topof dielectric layer 592 to the top of body region 556. A layer ofP⁺-doped polysilicon is then deposited over dielectric layer 592,filling the via holes to form conductive pillars (e.g., conductivepillars 591). This layer of P⁺-doped polysilicon is then patterned andetched to form conductors (e.g., conductor 590) to connect through vias593 to voltage source 594, which provides body bias voltage V_(bb). Bodybias voltage V_(bb) can be a positive high voltage applied during eraseor a low negative substrate bias voltage applied during read to raisethe TFT threshold voltage or reduce its sub-threshold leakage. FIG. 6eis a top view showing P⁺-doped polysilicon features 690-1 and 690-2formed.

In the embodiment shown in FIG. 5b , conductor 590 is provided abovebody region 556. In other embodiments, however, conductor 590 may beprovided underneath body region 556 to contact body region 556 frombelow. In fact, it may be advantageous to provide a body bias voltagefrom both above body region 556 and from below. In case of providing abody bias voltage from below, a conductor similar to conductor 590 maybe provided or directly from the substrate through a via in theinterlayer dielectric, similar to that shown in FIG. 5 a.

Modes of Operation of Segmented Local Bit Line and Segmented LocalSource Line Arrays.

In a memory stack of say 64 planes of word lines with bit line segments,such as described above with respect to the embodiments of the currentinvention, when reading a storage transistor on any plane (e.g., plane25) associated with a selected bit line segment, all word lines at allplanes that are associated with the selected bit line segment are heldat their “off” threshold voltage, except for the word line on theselected plane that is addressing the selected storage transistor. Whenthe word line voltage is brought up, a storage transistor that is in theerased state (i.e. conducting or “on” state) will discharge its bit linevoltage (V_(bl)) to its local source line (and its associated sourceline segment, if applicable), which has previously been pre-charged tovirtual ground potential (V_(ss)). The rate of discharge of bit linevoltage V_(bl) is sensed by the sense amplifier for the bit linesegment. Other storage transistors on the selected plane (i.e., 25^(th)plane, in this example) that are associated with other bit line segmentsalong the Y-direction sharing the same word line, or other storagetransistors associated with other bit line segments along theX-direction that are addressed by different word lines, can be readconcurrently, since each bit line segment has its dedicated senseamplifier. For a read operation, the virtual source voltage is firstpre-charged by setting the local bit line to 0V during the pre-chargeoperation. (Alternatively, the virtual source voltage can be elevated to˜1V.) After the pre-charge, the local bit line is charged to the senseamplifier voltage (e.g., at ˜0.1V to 0.5V higher than the sourcevoltage), the substrate is set to voltage V_(bb) (e.g., ˜0V to ˜−2V) andword line WL is raised to ˜1V-3V above the erase threshold voltage.

For embodiments in which the storage transistors on both sides of eachword line (e.g., embodiments of FIGS. 6a and 6e ), care must be taken toensure that only one of the two storage transistors is conducting at anytime during a read operation. This is achieved, as discussed above, byproviding separate bit line segments running parallel to each other, buteach being served by their own sense amplifiers, decoders, voltagesources and other support circuitry. As shown in FIG. 6e , the bit linesegments are MSBL₁(L) for the left-side storage transistors and MSBL₁(R)for the right-side storage transistor.

To program a storage transistor, all word lines on all planes except theselected plane (i.e., 25^(th) plane, in this example) are set at groundpotential, while the word line addressing the selected storagetransistor (i.e., on the 25^(th) plane) is raised to a suitableprogramming voltage using, for example, incremental voltage steps (e.g.,starting at ˜8 volts and applying voltage pulses of increasing magnitudein incremental steps) until the desired programmed voltage is verifiedby a read operation to have been reached. During the programmingoperation, the voltage on the bit line segment is held at groundpotential, as is the associated source line segment.

To inhibit further programming while continuing to program storagetransistors on the selected plane that are associated with other bitline segments sharing the same word line, the bit line segment and thesource line segment are raised to a program-inhibit voltage (e.g.,around one third to one half of the programming voltage), until the endof the programming sequence, with read verify cycles in-betweensuccessive programming pulses. All program and program inhibit voltagesto the local bit lines and the local source lines within a bit line orsource line segment are provided solely through the bit line segment(through pre-charge operation for the source line). As with the readoperation, storage transistors associated with other bit line segmentsalong the Y-direction (i.e., sharing the same word line as the selectedstorage transistor), and storage transistors associated with other bitline segments along the X-direction (i.e., associated with differentword lines), can be programmed or program-inhibited concurrently.

An erase operation may be accomplished by holding all word lines forstorage transistors associated with the bit line segments, the sourceline segments, or blocks to be erased at 0V, while raising the body biasvoltage (V_(bb)) to ˜12V for virgin storage transistors (i.e., storagetransistors that have never been programmed or erased), and up to 20V orhigher for high cycle-count storage transistors. All sense amplifiersassociated with a bit line segment may be isolated from their bit linesor bit line segments, as the floating N+ vertical local source lines andN+ vertical local bit lines within the erase block follow the positivevoltage applied to their p− body regions.

It is possible to read, program, program-inhibit and erase through otherconditions familiar to a person of ordinary skill in the art.

Low-Latency Partitioned Local and Global Word Lines.

The bit line segmentation in embodiments of the present invention servesto significantly reduce the RC delays in conventional global bit linesof conventional 3D NAND and 3D NOR memory arrays. Another majorcontributor to long read latency are the long and highly capacitivelocal word line conductors that typically run almost half or the entirewidth of the chip, orthogonal to the global bit lines. Thus, the 3Dvertical NOR Flash memory arrays of US 2017/0092371 A1, likeconventional 3D NAND Flash memory arrays, require a minimum of one layerof local word line conductors for each memory plane. In a 64-plane NANDor NOR memory array, these word line conductors are constructed in tallstair-case steps. Because local word lines supply high voltage duringprogramming, their decoders require high voltage transistors circuitrythat can occupy significant silicon real estate for each such stair-casestep.

To reduce their associated overhead cost, word lines are typically madeto be very long, which translates into high RC delays and poor readlatency (e.g., in the range of a few microseconds). In a conventional 3DNAND memory array, the global bit lines too are long and have slow riseor fall times, which essentially hides the long word line latency. Withthe bit line segments of the present invention, since the bit-lineresponse time can be very short (e.g., in the range of 100 nanoseconds),long word line RC delays become the limiting factor to fast read access.According to one embodiment of the present invention, one partialsolution makes the 3D NOR memory chip long and narrow (i.e, short alongthe direction of the word lines and long along the direction of the bitline segments). While such a design does not reduce the silicon area forforming the word line decoders, the lengths and the RC delays of theword lines are significantly reduced without significantly increasingthe RC delays along the bit line segments.

According to another embodiment of the present invention, word linedelays may be further reduced by partitioning the memory array into moreblocks with shorter word lines, each formed with its repeat stair-casesteps. Partitioning the memory arrays by doubling the number ofstair-case steps and their word line decoders reduce the RC delays by4-fold.

Another significant contributor to long read latency is the large RCdelays of the global word lines (GWL) that run in the X-directionspanning the length of the memory array above the stair-case steps alongthe sides of the memory array. FIG. 6f illustrates one implementation ofglobal word lines for connecting the local word lines on one plane(i.e., at one stair-case step) in conjunction with the bit linesegmentation scheme of the present invention. In FIG. 6f , only thelocal word lines at one X-Y plane through a stair-case step along theside of a memory array, the global word lines above the stair-case stepsand their interconnections are shown. For clarity of illustration, allother details (e.g., P⁻ channel material layers and charge trappinglayers) are omitted. As shown in FIG. 6f , word lines WL₀, WL₁, . . . ,of the memory array (e.g., the memory array corresponding to theembodiment shown in FIG. 6e ) run along the Y-direction. Global wordlines GWL₀, GWL₁, . . . , run along the X-direction above the stair-casesteps. The global word lines connect the word lines at each plane of thememory array to their respective decoders, voltage sources and othersupport circuitry in substrate 605. In applying bit line segmentation tothe architecture, for example, of FIGS. 3d, 3e, 3f and 3g , each step inthe stair-case accommodates up to n global word lines that matches thenumber n of local word lines within a bit line segment. In theembodiment of FIG. 6f , for example, each bit line segment may include128 bit lines and each storage transistor at each step is selected by acorresponding word line. Thus, there are 128 word lines at each step ofa bit line segment. Hence, each global word line connects to every128^(th) word line. For example, on each plane, global word line GWL₀connects to word lines WL-0, WL-127, . . . through vias VIA₀, VIA₁₂₇, .. . , and GWL₁ connects to word lines WL-1, WL-129, . . . through viasVIA₁, VIA₁₂₈, . . . to its substrate decoders and voltage sources insubstrate 605. This arrangement allows 128 sets of storage transistorson each plane to be concurrently read by activating the common globalword line and their dedicated sense amp decoders. For example, storagetransistors associated with word lines WLi, WL_(i+128), . . .(generally, WL_(i+128k), k=0, 1, . . . ) may be simultaneously read orprogrammed by activating global word line GWLi, while all other globalword lines at the same step and at the other steps can be at groundpotential (i.e all other storage transistors off) or floated at groundpotential.

The embodiment illustrated in FIG. 6f may be considered costly insilicon real estate: if there are 128 word lines in each bit linesegment and 64 steps in the stair-case, 128 global word lines would berequired for every step of a 64-step stair-case (or 8192 global wordlines in total). According to one embodiment of the present invention,the number of global word lines required can be reduced by a factor of2, 4, 8, 16 or more by having each global word line contact more thanone local word line within each bit-line segment. For example, globalword line GSL₁ may contact not only word line WL₁, WL₁₂₉, . . . but alsoword lines WL₃₃, WL₆₅, . . . , (generally, WL_(1+32k), k=0, 1, . . . )thereby reducing by a factor of four the number of global word linesrequired per step, and reducing by a factor of four the total width ofthe stair-case. Of course, either additional decoding circuitry, or fourtimes the number of dedicated sense amplifiers for each bit linesegment, are required in the silicon substrate. (Alternatively, thesingle sense amplifier of the bit line segment may be time-sharedthrough four consecutive read or program sequences.)

As the global word lines are implemented at the top of the memory arrayabove the stair-case steps, the global word lines can be implementedusing low resistivity copper interconnects. Capacitance between adjacentglobal word lines within a step can be reduced by substitution air gapsas the dielectric between them, as known to a person of ordinary skillin the art. The global word line RC delays can be reduced further byconnecting global word line decoders and voltage sources in the siliconsubstrate underneath the stair-case steps to access the global wordlines every half, quarter or eighth of their length through breaks alongthe length of the global word lines.

When going from say, a 32-layer stack to a 64-layer stack, the number ofword line stair-case steps is doubled from 32 to 64. FIG. 6g shows animplementation of a vertical NOR string memory array that avoids suchstep-doubling, according to one embodiment of the present invention. InFIG. 6g , a Z-Y cross section of a memory array is shown with the totalnumber of planes in the memory array being provided as two or moresuccessively formed stacks (e.g., STK₁ and STK₂), one on top of another.Each stack is provided its own set of stair-case steps completed beforethe next stack is formed. In 3-dimensional NAND memory arrays of theprior art, two stacks of memory cells, each of 32 planes, are formed.Thereafter, a 64-plane stair-case of steps are then formed separately,followed by forming their associated global word lines. In contrast,FIG. 6f shows forming stacks STK₁ and stack STK₂ each having just 32stair-case wide steps (Steps A, Steps B), each step being a word line(running along the Y-direction) connected by one of global word lineGWL₁, GWL₂, . . . , GWL₃₂ (running along the X-direction). Stacks STK1and STK2 are isolated from each other by isolation layer 617, thusreduced in half the total width of providing 64 stair-case steps. Underthis scheme, local bit line (e.g., BL 654) and local source line (e.g.SL 655) in stack STK₂ are connected to their corresponding local bitline and local source line in stack STK₁ by etching openings throughisolation layer 617 to expose the top of the N+ doped vertical columns,thereby connecting the vertical active columns of the top 32 planes totheir counterparts in the lower 32 planes above substrate 605. Likewise,P− doped channel regions (e.g., channel region 656, corresponding tochannel region 556 of FIG. 5b ) of both stacks STK₁ and STK₂ areconnected together by P+ doped plugs 691, which is formed in isolationlayer 617 prior to forming STK₂.

The silicon substrate area associated with the global word lines can bereduced by positioning the global word line decoders and voltage sourceseither below the stair-case steps or on top of the memory arrays ratherthan outside of the arrays in the substrate. Such placement may beprovided in conjunction with memory arrays of FIGS. 3f and 3g . In thoseembodiments, the top surface of the memory array is clear of any sourceline or bit line interconnects. Of course, such word line decoders andvoltage sources are implemented using thin-film transistors that must beable to support the relatively high voltages (e.g., in the range of12V-20V) required on the global word lines during programming Suchthin-film transistors may be achieved through shallow (Excimer) laseranneal to partially recrystallize deposited polysilicon or through otherseeding techniques developed for solar panels or LED displays or otherapplications. The top surface of the memory array can also be exploitedto run wider or taller global word line interconnects with greaterspacing in-between to reduce their RC delays without unduly increasingthe memory chip area.

3D Vertical NOR Arrays with Segmented Bit Lines for Quasi-Volatile NORStrings.

Non-provisional Patent Application III, which is incorporated byreference above and which is now published as US 2017/0092371A1 (“the'237 publication”), discloses quasi-volatile NOR strings (see the '237publication, at paragraphs [0128]-[0131]) that are suitable forreplacing DRAM in certain storage applications that do not requireextremely high cycle endurance. To that end, the read access time ofquasi-volatile NOR strings approaches the read access time of DRAM,which, at under 100 nanoseconds, is approximately 500 times faster thanconventional 3D NAND flash memory. In the three-dimensional vertical NORstrings disclosed in this detailed description, the segmented bit-linesat the bottom of the array with their dedicated sense amplifiers,decoders in the substrate beneath the bit line segment (e.g., shown inFIGS. 3d, 3e, 3f, and 3g ) closely emulate the horizontal strings ofNon-Provisional Patent Application III and are equally capable ofachieving near-DRAM read latency. The process steps for building thesequasi-volatile vertical NOR strings are similar to the steps describedat paragraph [0129] of '237 publication. Because of the relatively shortretention time (e.g., in the range of one hour to a few days) of thequasi-volatile storage transistors, they need to be frequentlyread-refreshed; in that context, having the ability to read or reprograma large number of storage transistors concurrently (i.e. reading andreprogramming storage transistors associated with many bit line segmentsin parallel) is critical for minimizing interruption of normal readswhen chip densities approach 1-terabit.

Non-Provisional Patent Application III also discloses pairing twostorage transistors for a fast-read cache memory in horizontal NORstrings (see, the '237 publication, at paragraphs [0194]-[0196]). Thesegmented bit line with dedicated segment sense amplifier in thevertical NOR strings, as disclosed in this detailed description, iswell-suited for such fast read cache memory, wherein a dual transistorpair may be used to program data on one transistor and the inverse data(i.e. the erased state) on an adjacent transistor sharing the same wordline. For example, in FIG. 6e , the read output signals from the twotransistors T_(L) (683), T_(R) (682) in two adjacent bit-line segmentsMSBL₁(L), MSBL₁(R) sharing the two sides of the same word line WL₃₁₋₁are fed into a differential sense amplifier in the silicon substrate.The differential sense amplifier is shared between the two adjacent bitline segments along the Y-direction. This dual segment arrangement,although it cuts by 50% the array bit efficiency, provides superiorimmunity to process variations and string leakage, parameter drifts ordevice sensitivities across the chip, while providing very fast sensing,higher cycle endurance, and dispensing with the need for programmablereference strings. Because of the isolation between bit line segmentsalong the X-direction (i.e., along the same direction as the global bitlines), it is possible to have on the same chip blocks of bit linesegments that are configured with paired transistors differentialsensing for cache storage while other blocks employ the regular sensingof single transistors at a time for double density. This flexibilityallows the same chip to serve as partially a cache memory, part astorage memory. It also allow storing files that require many pages ofstorage (e.g., one photo image requiring 4 MB of storage occupies 2,000pages each of 2 KB) to have their first one or more pages written intothe segments with fast cache memory and the rest in the non-cachesegments on the same chip, then retrieve the image by reading its firstpage very fast, while employing pipeline reads for the other pages toenjoy the lower read latency for the entire 4 MB.

Although the segmentation of a global bit line into regional bit linesegments with corresponding segment sense amplifiers and the global wordline segmentation (discussed in conjunction with FIGS. 6f and 6h ) ofthe present invention have been described for 3-dimensional vertical NORstrings, it can be similarly applied to conventional 3D vertical NANDmemory strings.

Fabrication Process

FIGS. 7a, 7b, 7c and 7d are cross sections of intermediate structuresformed in a fabrication process for a multi-gate NOR string array, inaccordance with one embodiment of the present invention.

FIG. 7a shows a cross section in the Z-Y plane of semiconductorstructure 700, after low resistivity layers 723 p have been formed abovesubstrate 701, in accordance with one embodiment of the presentinvention. In this example, p is an integer between 0 and 31,representing each of 32 word lines. As shown in FIG. 7a , semiconductorstructure 700 includes low resistivity layers 723-0 to 723-31.Semiconductor substrate 701 represents, for example, a P− doped bulksilicon wafer on and in which support circuits for memory structure 700may be formed prior to forming the vertical NOR strings. Such supportcircuits may include both analog and digital logic circuits. Someexamples of such support circuits may include shift registers, latches,sense amplifiers, reference cells, power supply lines, bias andreference voltage generators, inverters, NAND, NOR, Exclusive-OR andother logic gates, input/output drivers, address decoders, includingbit-line and word line decoders, other memory elements, sequencers andstate machines. To provide these support circuits, the building blocksof conventional N-Wells, P-Wells, triple wells (not shown), N⁺ diffusionregions (e.g., region 707-0) and P⁺ diffusion regions (e.g., region706), isolation regions, low and high voltage transistors, capacitors,resistors, diodes and interconnects are provided, as known to a personskilled in the art.

After the support circuits have been formed in and on semiconductorsubstrate 701, insulating layers 708 are provided, which may bedeposited or grown thick silicon dioxide, for example. In someembodiments, one or more metallic interconnect layers may be formed,including global source line 713-0, which may be provided as horizontallong narrow strips running along a predetermined direction. Globalsource line 713-0 is connected through etched openings 714 to circuitry707 in substrate 701. To facilitate discussion in this detaileddescription, the global source lines are presumed to run along theX-direction. The metallic interconnect lines may be formed by applyingphoto-lithographical patterning and etching steps on one or moredeposited metal layers. (Alternatively, these metallic interconnectlines can be formed using a conventional damascene process, such as aconventional copper or tungsten damascene process). Thick dielectriclayer 709 is then deposited, followed by planarization usingconventional chemical mechanical polishing (CMP).

Conductor layers 723-0 to 723-31 are then successively formed, eachconductor layer being insulated from the layer underneath it and thelayer above it by an intervening insulating layers 726. In FIG. 7a ,although thirty two conductor layers are indicated, any number of suchlayers may be provided. In practice, the number of conductor layers thatcan be provided may depend on the process technology, such as theavailability of a well-controlled anisotropic etching process thatallows cutting through the multiple conductor layers and dielectricisolation layers 726 there-between. For example, conductor layers 723 pmay be formed by first depositing 1-2 nm thick layer of titanium nitride(TiN), followed by depositing a 10-50 nm thick layer of tungsten (W) ora similar refractory metal, or a silicide such as silicides of nickel,cobalt or tungsten among others, or a salicide, followed by a thin layerof etch-stop material such as aluminum oxide (Al₂O₃). Each conductorlayer is etched in a block 700 after deposition, or is deposited as ablock through a conventional damascene process. In the embodiment shownin FIG. 7a , each successive conductor layer 723 p extends in the Y−direction a distance 727 short of (i.e. recessed from) the edge of theimmediately preceding metal layer, so that all conductor layers may becontacted from the top of structure 700 at a later step in the process.However, to reduce the number of masking and etch steps necessary toform the stepped conductors stack of FIG. 7a , it is possible to achieverecessed surfaces 727 simultaneously for multiple conductor layers byemploying other process techniques known to a person skilled in the artthat do not require each individual conductor plane to be separatelymasked and etched to create exposed recessed surfaces 727. After theconductor layer is deposited and etched, the corresponding one ofdielectric isolation layers 726 is then deposited. Dielectric isolationlayers 726 may be, for example, a silicon dioxide of a thickness between15 and 50 nanometers. Conventional CMP prepares the surface of eachdielectric layer for depositing the next conductor layer. The number ofconductor layers in the stack of block 700 corresponds to at least thenumber of memory TFTs in a vertical NOR string, plus any additionalconductor layers that may be used as control gates of non-memory TFTssuch as pre-charge TFTs (e.g., pre-charge TFT 575 of FIG. 5a ), or ascontrol gates of bit-line access select TFTs (e.g., 585 bit-line accessselect TFT 511 of FIG. 5a ). The conductor layer deposition and etchsteps and the dielectric layer deposition and CMP process are repeateduntil all conductor layers are provided.

Dielectric isolation layer 710 and hard mask layer 715 are thendeposited. Hard mask 715 is patterned to allow etching of conductorlayers 723 p to form long strips of yet to be formed word lines. Theword lines extend in length along the Y-direction. One example of amasking pattern is shown in FIG. 6 for word lines 623 p-R, 623 p-L,which includes features such as the extensions in adjacent word linestowards each other at separation 676 and the recesses in each word lineto create the desired curvatures 675. Deep trenches are created byanisotropically etching through successive conductor layers 723 p andtheir respective intervening dielectric insulator layers 726, untildielectric layer 709 at the bottom of conductor layers 723 p is reached.As a large number of conductor layers are etched, a photoresist mask byitself may not be sufficiently robust to hold the desired word linepattern through numerous successive etches. To provide a robust mask,hard mask layer 715 (e.g., carbon) is preferred, as is known to a personof ordinary skill in the art. Etching may terminate at dielectricmaterial 709, or at landing pads 713 on the global source lines, or atsubstrate 701. It may be advantageous to provide an etch-stop barrierfilm (e.g., aluminum oxide) to protect landing pads 713 from etching.

FIG. 7b illustrates, in a cross section in the Z-X plane ofsemiconductor structure 700, etching through successive conductor layers723 p and corresponding dielectric layers 726 to form trenches (e.g.,deep trench 795), which reach down to dielectric layer 709, according toone embodiment of the present invention. In FIG. 7b , conductor layers723 p are anisotropically etched to form conductor stacks 723 p-R and723 p-L, which are separated from each other by deep trench 795. Thisanisotropic etch is a high aspect-ratio etch. To achieve the bestresult, etch chemistry may have to be alternated between conductormaterial etch and dielectric etch, as the materials of the differentlayers are etched through, as in known to a person skilled in the art.The anisotropy of the multi-step etch is important, as undercutting ofany of the layers should be avoided, so that a resulting word line atthe bottom of a stack would have approximately the same conductor widthand trench spacing as the corresponding width and spacing of a word linenear or at the top of the stack. Naturally, the greater the number ofconductor layers in the stack, the more challenging it becomes tomaintain a tight pattern tolerance through the numerous successiveetches. To alleviate the difficulty associated with etching through, forexample, 64 or 128 or more conductor layers, etching may be conducted insections of, say, 32 layers each. The separately etched sections canthen be stitched together, as taught, for example, in the Kim referencementioned above.

Etching through multiple conductor layers 723 p of conductor material(e.g., tungsten or other refractory materials) is much more difficultand time-consuming than etching of the intervening insulating layers726. For that reason, an alternative process may be adopted thateliminates the need for multiple etches of conductor layers 723 p. Thatprocess, well known to a person skilled in the art, consists of firstsubstituting sacrificial layers of a readily etchable material in placeof conductor layers 723 p of FIG. 7b . For example, insulating layers726 can be silicon dioxide and sacrificial layers (occupying the spacesshown as 723 p in FIG. 7b ) can be silicon nitride or another fastetching dielectric material. Deep trenches are then etchedanisotropically through the ONON (Oxide-Nitride-Oxide-Nitride)alternating dielectric layers to create tall stacks of the dualdielectrics. At a later step in the manufacturing process flow (to bedescribed below), these stacks are supported by active vertical stripsof polysilicon, allowing the sacrificial layers to be etched away,preferably through selective chemical or isotropic etch. The cavitiesthus created are then filled through conformal deposition of theconductor material, resulting in conductor layers 723 p separated byintervening insulating layers 726.

After the structure of FIG. 7b is formed, charge-trapping layers 734 andpolysilicon layers 730 are then deposited in succession conformally onthe vertical sidewalls of the etched conductor word line stacks. A crosssection in the Z-X plane of the resulting structure is shown in FIG. 7c. As shown in FIG. 7c , charge-trapping layers 734 are formed, forexample, by first depositing blocking dielectric 732 a, between 5 to 15nanometers thick and consisting of a dielectric film of a highdielectric constant (e.g., aluminum oxide, hafnium oxide, or somecombination silicon dioxide and silicon nitride). Thereafter,charge-trapping material 732 b is deposited to a thickness of 4 to 10nanometers. Charge-trapping material 732 b may be, for example, siliconnitride, silicon-rich oxynitride, conductive nanodots embedded in adielectric film, or thin conductive floating gates isolated fromadjacent TFTs sharing the same vertical active strip. Charge-trapping732 b may then be capped by a deposited conformal thin tunnel dielectricfilm in the thickness range of 2 to 10 nanometers (e.g., a silicondioxide layer, or a silicon oxide-silicon nitride-silicon oxide (“ONO”)triple-layer). The storage element formed out of charge-trapping layers734 may be any one of SONOS, TANOS, nanodot storage, isolated floatinggates or any suitable charge-trapping sandwich structures known to aperson of ordinary skill in the art. The combined thickness ofcharge-trapping layers 734 is typically between 15 and 25 nanometers.

After deposition of charge-trapping layer 734, contact openings are madeat the bottom of trench 795, using a masking step and by anisotropicallyetching through charge-trapping layers 734 and dielectric layer 709 atthe bottom of trench 795, stopping at bottom global source line landingpad 713 for the source supply voltage V_(ss) (see, FIG. 7b ), or atglobal bit line voltage V_(bl) (not shown), or at P+ region 706 forcontact to a back bias supply voltage V_(bb) (see, FIG. 7c ). In someembodiments, this etch step is preceded by a deposition of an ultra-thinfilm of polysilicon (e.g. 2 to 5 nanometers thick) to protect thevertical surfaces of tunnel dielectric layer 732 c during thecontact-opening etch of charge-trapping material 734 at the bottom oftrench 795. In one embodiment, each global source line is connected onlyto alternate ones in a row of vertical NOR string pairs. For example, inFIG. 5a , for odd address word lines, electrical contacts (e.g., contactopening 557) are etched to connect the N+ doped local source lines(e.g., local source line 555 in FIG. 5a ) to global source line 513-1.Likewise, for even address word lines, electrical contacts are etched toconnect the N+ doped local source lines in the row of vertical NORstring pairs to global source line 513-2 (not shown in FIG. 5a ). In theembodiment employing virtual V_(ss) through parasitic capacitor C (i.e.,capacitors 560 in FIG. 5a ) the step of etching through charge trappinglayer 734 at the bottom of trench 795 may be skipped.

Thereafter, polysilicon thin film 730 is deposited to a thicknessranging between 5 and 10 nanometers. In FIG. 7c , polysilicon thin film730 is shown on the opposite sidewalls of trench 795, labeledrespectively 730R and 730L. Polysilicon thin film 730 is undoped orpreferably doped P− with boron, at a doping concentration typically inthe range of 1×10¹⁶ per cm³ to 1×10¹⁷ per cm³, which allows a TFT to beformed therein to have an enhancement native threshold voltage. Trench795 is sufficiently wide to accommodate charge-trapping layers 734 andpolysilicon thin film 730 on its opposing sidewalls. Following thedeposition of polysilicon 730, the sacrificial layers in the stackdescribed above are etched away and the cavities thus formed are filledwith the conformally deposited conductor layers 723 p (FIG. 7c ).

As shown in FIG. 7b , trench 795 extends along the Y-direction. Afterformation of isolated word line stacks 723 p-L and 723 p-R, in oneexample semiconductor structure 700 may have 16,000 or more side-by-sideword line stacks, each serving as control gates for 8,000 or more activecolumns to be formed along the length of each stack, or 16,000 TFTs(8,000 TFTs on each side of the stack). With 64 word lines in eachstack, 16 billion TFTs may eventually be formed in each of suchmulti-gate vertical NOR string array. If each TFT stores two data bits,such a multi-gate vertical NOR string array would store 32 gigabits ofdata. Approximately 32 such multi-gate vertical NOR string arrays (plusspare arrays) may be formed on a single semiconductor substrate, therebyproviding a 1-terabit integrated circuit chip.

FIG. 7d is a cross section view in the X-Y plane of the top surface ofthe structure of FIG. 7c in one embodiment. Nestled between word lines723 p-L and 723 p-R are the two sidewalls 730L and 730R of the verticaldeposited P− doped polysilicon structure (i.e., an active column). Thedeep void 740 between sidewalls 730L and 730R may be filled with afast-etching insulating dielectric material (e.g., silicon dioxide orliquid glass or carbon doped silicon oxide). The top surface may then beplanarized using conventional CMP. A photolithographic step then exposesopenings 776 and 777, which is followed by a high aspect-ratio selectiveetching to excavate the fast-etching dielectric material in exposedareas 776 and 777 all the way down to the bottom of trench 795. A hardmask may be required in this etching step to avoid excessive patterndegradation during etch. The excavated voids are then filled with anin-situ N+ doped polysilicon. The N+ dopants diffuse into the very thinlightly doped active polysilicon pillars 730L and 730R within theexposed voids to make them N+ doped. Alternatively, prior to filling thevoids with the in-situ N+ doped polysilicon the lightly dopedpolysilicon inside the voids can be etched away through a briefisotropic plasma etch or selective wet etch. CMP or top surface etchingthen removes the N+ polysilicon from the top surface, leaving tall N+polysilicon pylons in areas 754 (N+) and 755(N+). These N+ pylons formthe shared vertical local source line and the shared vertical local bitline for the TFTs in the resulting vertical NOR strings.

FIG. 7d -1 shows materially enhancing electrical conductivity of thetall vertical source/drain pylons by only partially filling the exposedvoids 776 of vertical pylons 754 and 755, for example, by firstdepositing ultra-thin layers of N+ doped polysilicon 754(N+) and755(N+), each of thickness between 5 and 15 nanometers (which isinsufficient to fill the voids), followed by depositing (e.g., usingAtomic Layer Deposition (ALD)) a metallic conductive material (e.g.,titanium nitride, tungsten nitride or tungsten) to fill remaining void720(M) at the core of the source/drain pylons. See, also, FIG. 4a -1,which shows in the Y-Z plane metallic conductor 420(M) occupying thecore of the pylons, in close contact with ultra-thin N+ poly 454 (N+).Because of the relatively significantly higher conductivity of themetallic material at the core, the N-type doping concentration of theultra-thin N+ doped polysilicon can be reduced by one or two orders ofmagnitude, reducing undesirable thermal diffusion of the N-type dopantinto the P-type dopant of the channel. The N+/metallic conductorstructure can be applied to either one or both of the source and drainpylons. In another embodiment, the thin P− doped polysilicon that is inregion 757—outside the channel region 756—can first be more heavilydoped P+(e.g., 10¹⁹ per cm³ or higher), compared to the P− doping inchannel region 756, which may be 2×10¹⁸ per cm³ or lower. Adding the P+poly in the source pylon that contacts the P− poly in the channel canenhance erase efficiency when the local source line is raised to a highpositive voltage during an erase operation.

Next, a dielectric isolation layer is deposited and patterned usingphotolithographic masking and etching steps. The etching step openscontacts for connecting the vertical local bit lines to the horizontalglobal bit lines (e.g., contacts 657-1 to strings at odd addresses and657-2 to strings at even addresses, as shown in FIG. 6). A lowresistivity metal layer (e.g., tungsten) is deposited. The depositedmetal is then patterned using photolithographic and etching steps toform global bit-lines (e.g., global word line 614-1 or GBL1 for stringsat odd addresses, and global bit line 614-2 (GBL2) for strings at evenaddresses, as shown in FIG. 6). Alternatively, the global bit lines maybe formed using conventional copper damascene process. All global bitlines, as well as all metal layers 723 p of the word line stacks (FIG.7a ) are connected by etched vias to word line and bit-line decoding andsensing circuits in the substrate, as is known to a person skilled inthe art. Switch and sensing circuits, decoders and reference voltagesources can be provided to global bit lines and global word lines,either individually or shared by several ones of the bit lines and wordlines.

In some embodiments, bit line access select transistors (511 in FIG. 5a) and their associated control gate word lines (e.g., word lines 585 inFIG. 5a ) are formed as isolated vertical N+P−N+ transistors, as knownto a person skilled in the art, to selectively connect odd and evenglobal bit lines (e.g., bit lines 614-1 and 614-2 in FIG. 6a ) tovertical NOR strings at alternate odd and even addresses (e.g., localbit lines 657-1 and 657-2, respectively, in FIG. 6a ).

Read Operation

Because the TFTs of a vertical NOR string are connected in parallel, inall embodiments of the current invention, all TFTs in an active column(including an active column having formed thereon a vertical NOR stringpair) should preferably be in enhancement mode—i.e., each TFT shouldhave a positive gate-to-source threshold voltage—so as to suppressleakage currents during a read operation between the shared local sourceline and the shared local bit line (e.g., local bit line 455 and localsource line 454 shown in FIG. 4c ). Enhancement mode TFTs are achievedby doping the channel regions (e.g., P-channel region 756 of FIG. 7c )with boron in a concentration typically between 1×10¹⁶ and 1×10¹⁷ percm³, targeting for a native TFT threshold voltage of around 1V. Withsuch TFTs, all unselected word lines in the vertical NOR string pair ofan active column may be held at 0V. Alternatively, the read operationmay raise the voltage on the shared local N+ source line (e.g., localsource line 455 of FIG. 4c ) to around 1.5V, while raising the voltageon the shared local N+ drain line (e.g., local bit line 454) to around2V and holding all unselected local word lines at 0V. Such aconfiguration is equivalent to setting the word line to −1.5V withrespect to the source, thereby suppressing leakage current due to TFTsthat are in slightly depleted threshold voltage, which occurs, forexample, if the TFTs are slightly over-erased.

After erasing the TFTs of a vertical NOR string, a soft programmingoperation may be required to shift any TFT in the vertical NOR stringthat is over-erased (i.e., now having a depletion mode thresholdvoltage) back to an enhancement mode threshold voltage. In FIG. 5a , anoptional connection 556 is shown by which P− channel is connected toback bias voltage 506 (V_(bb),) (also shown as body connection 456 inFIG. 4c ). A negative voltage may be used for V_(bb) to modulate thethreshold voltage of the TFTs in each active column to reducesubthreshold leakage currents between the shared N+ source and theshared N+ drain/local bit line. In some embodiments, a positive V_(bb)voltage can be used during an erase operation to tunnel-erase TFTs whosecontrol gates are held at 0V.

To read the data stored in a TFT of a vertical NOR string pair, all TFTson both vertical NOR strings of the vertical NOR string pair areinitially placed in the “off” state by holding all word lines in themulti-gate NOR string array at 0V. The addressed vertical NOR string caneither share a sensing circuit among several vertical NOR strings alonga common word line through use of decoding circuitry. Alternatively,each vertical NOR string may be directly connected through a globalbit-line (e.g., GBL1 of FIG. 4c ) to a dedicated sensing circuit. In thelatter case, one or more vertical NOR strings sharing the same word lineplane may be sensed in parallel. Each addressed vertical NOR string hasits local source line set at V_(ss)˜0V, either through its hard-wiredglobal source line (e.g., GSL1 in FIG. 4c ) as shown schematically inFIG. 8a , or as a virtual V_(ss)˜0V through a pre-charge transistor(e.g., pre-charge transistor 470 in FIG. 4c or transistor 317 in FIG. 3c) which momentarily transfers V_(bl)˜0V to parasitic capacitance C(e.g., capacitor 460 or capacitor 360) of floating local source line 455or 355) during the pre-charge, as shown schematically in FIG. 8 b.

Immediately after turning off pre-charge transistor 470, the local bitline (e.g., local bit line 454 of FIG. 4c ) is set at V_(bl)˜2V throughthe bit line access select transistor (e.g., bit line access selecttransistor 411 of FIG. 4c or access select transistor 511 in FIG. 5a ).V_(bl)˜2V is also the voltage at the sense amplifiers for the addressedvertical NOR strings. At this time, the addressed word line is raised insmall incremental voltage steps from 0V to typically about 6V, while allthe un-selected word lines at both the odd address TFTs and the evenaddress TFTs of the vertical NOR string pair remain at 0V. In theembodiment of hard-wired V_(ss) of FIG. 8a , the addressed TFT has beenprogrammed in one example to a threshold voltage of 2.5V, therefore thevoltage V_(bl) at local bit line LBL begins to discharge through theselected TFT towards the OV of the local source line (V_(ss)) as soon asits WL_(S) exceeds 2.5V, thus providing a voltage drop (shown by thedashed arrow in FIG. 8a ) that is detected at the sense amplifierserving the selected global bit line. In the embodiment of the virtualV_(ss) of FIG. 8b , pre-charge transistor word line WL_(CHG) momentarilyis turned on to pre-charge floating local source line LSL to 0V at thestart of the read sequence. Then, selected word line WL_(S) goes throughits incremental voltage steps, and as soon as it exceeds the programmed2.5V, the selected TFT momentarily dips the voltage on its local bitline from its V_(bl)˜2V. This voltage dip (shown by the dashed arrow inFIG. 8b ) is detected by the sense amplifier of the global bit lineconnected to the selected local bit line. There are other alternativeschemes to correctly read the programmed threshold voltage of theselected TFT as known to a person skilled in the art. The embodimentsrelying on parasitic capacitance C to temporarily hold virtual voltageV_(ss), the higher the vertical stack the bigger is capacitance C andtherefore the longer is the hold time and the greater is the read signalpresented to the selected sense amplifier. To further increase C it ispossible to add in one embodiment one or more dummy conductors in thevertical string whose primary purpose is to increase capacitance C.

In the case of an MLC implementation (i.e., a “multi-level cell”implementation, in which each TFT stores more than one bit), theaddressed TFT may have been programmed to one of several voltages (e.g.,1V (erased state), 2.5V, 4V or 5.5V). The addressed word line WL_(S) israised in incremental voltage steps until conduction in the TFT isdetected at the sense amplifier. Alternatively, a single word linevoltage can be applied (e.g., ˜6 volts), and the rate of discharge ofthe local bit line LBL (V_(bl)) can be compared with the rates ofdischarge from several programmable reference voltages representative ofthe voltage states of the stored multi-bit. This approach can beextended for a continuum of states, effectively providing analogstorage. The programmable reference voltages maybe stored in dedicatedreference vertical NOR strings located within the multi-gate verticalNOR string array, so that the characteristics during read, program, andbackground leakage are closely tracked. In a vertical NOR string pair,only the TFTs on one of the two vertical NOR strings can be read in eachread cycle; the TFTs on the other vertical NOR string are placed in the“off” state (i.e., all word lines at 0V). During a read cycle, as onlyone of the TFTs in a vertical NOR string is exposed to the readvoltages, read disturb conditions are essentially absent.

In one example of an embodiment of this invention, 64 TFTs and one ormore pre-charge TFTs may be provided on each vertical NOR string of avertical NOR string pair. Each word line at its intersection with thelocal vertical N+ source line pillar forms a capacitor (see, e.g.,capacitor 660 of FIG. 6a ). A typical value for such a capacitor may be,for example, 1×10⁻¹⁸ farads. Including all the capacitors in bothvertical NOR strings of a vertical NOR string pair, the overalldistributed capacitance C totals approximately 1×10⁻¹⁶ farads, which issufficient for a local source line to preserve a pre-charged sourcevoltage (V_(ss)) during a read cycle, which is completed in typicallyless than a microsecond immediately following the pre-charge operation.The charging time through bit-line access select transistors 411 andpre-charge TFT 470 is in the order of a few nanoseconds, thus thecharging time does not add noticeably to the read latency. Reading froma TFT in a vertical NOR string is fast, as the read operation involvesconduction in only one of the TFTs in the vertical NOR string, unlikethe read operation on a NAND string, in which many TFTs connected inseries are required to be conducting.

There are two major factors contributing to the read latency of verticalNOR strings of the current invention: (a) the RC time delay associatedwith resistance R_(bl) and capacitance C_(bl) of a global bit line(e.g., GBL 614-1 in FIG. 6a ), and (b) the response time of a senseamplifier to a voltage drop V_(bl) on the local bit line (e.g., LBL-1)when the addressed TFT begins conducting. The RC time delay associatedwith a global bit line serving, for example, 16,000 vertical NOR stringsis of the order of a few tens of nanoseconds. The read latency forreading a TFT of a prior art vertical NAND string (e.g., the NAND stringof FIG. 1b ) is determined by the current through 32 or moreseries-connected TFTs and select transistors discharging capacitanceC_(bl) of the global bit line. By contrast, in a vertical NOR string ofthe present invention, the read current discharging C_(bl) is providedthrough just the one addressed transistor (e.g., transistor 416L of FIG.4a ) in series with bit line access select transistor 411, resulting ina much faster discharge of the local bit line voltage (V_(bl)). As aresult, a much lower latency is achieved.

In FIG. 4c , when one TFT (e.g., TFT 416L in the vertical NOR string 451b) is read at a time, all other TFTs in either vertical NOR string 451 aand 451 b of vertical NOR string pair 491 are held in their “off”states, their word lines being held at 0V. Even though TFT 416R invertical NOR string 452 a of vertical NOR string pair 492 shares wordline W31 with TFT 416L, TFT 416R may be read simultaneously with TFT416L because vertical NOR string 452 a is served by global bit line414-2, while vertical NOR string 451 b is served by global bit line414-1. (FIGS. 6a and 6b illustrate how global bit lines 614-1 and 614-2serve adjacent vertical NOR string pairs).

In one embodiment, a word line stack includes 32 or more word linesprovided in 32 planes. In one multi-gate vertical NOR string array, eachplane may include 8000 word lines controlling 16,000 TFTs, each of whichmay be read in parallel through 16,000 global bit lines, provided thateach bit line is connected to a dedicated sense amplifier.Alternatively, if several global bit lines share a sense amplifierthrough a decode circuit, the 16000 TFTs are read over severalsuccessive read cycles. Reading in parallel a massive number ofdischarging TFTs can cause a voltage bounce in the ground supply(V_(ss)) of the chip, which may result in read errors. However, anembodiment that uses the pre-charged parasitic capacitor C in the localsource line (i.e., providing a virtual source voltage (V_(ss)) forvertical NOR string) has a particular advantage in that such groundvoltage bounce is eliminated. This is because the virtual sourcevoltages in the vertical NOR strings are independent and are notconnected to the ground supply of the chip.

Program (Write) and Program-Inhibit Operations.

Programming of an addressed TFT may be achieved by tunneling—eitherdirect tunneling or Fowler-Nordheim tunneling,—of electrons from thechannel region of the TFT (e.g., channel region 430L shown in FIG. 4b )to the charge-trapping layer (e.g., charge trapping layer 434) when ahigh programming voltage is applied between the selected word line(e.g., word line 423 p-R) and the active channel region (e.g., activechannel region 456 in FIG. 4a ). As tunneling is highly efficient,requiring very little current to program a TFT, parallel programming oftens of thousands of TFTs may be achieved at low power dissipation.Programming by tunneling may require, for example, a 20V,100-microsecond pulse. Preferably, the programming is implementedthrough a succession of shorter duration stepped voltage pulses,starting at around 14V and going as high as approximately 20V. Steppedvoltage pulsing reduces electrical stress across the TFT and avoidsovershooting the intended programmed threshold voltage.

After each programming high-voltage pulse the addressed transistor isread to check if it has reached its target threshold voltage. If thetarget threshold voltage has not been reached, the next programmingpulse applied to the selected word line is incremented typically by afew hundred millivolts. This program-verify sequence is repeatedlyapplied to the one addressed word line (i.e., a control gate) with 0Vapplied to the local bit line (e.g., local bit line 454 of FIG. 4a ) ofthe active column (e.g., column 430L of FIG. 4b ). At these programminghigh word line voltages, TFT 416L's channel region is inverted and isheld at 0V, so that electrons tunnel into the charge storage layer ofTFT 416L. When the read sensing indicates that the addressed TFT hasreached its target threshold voltage, the addressed TFT must beinhibited from further programming, while other TFTs sharing the sameword line may continue programming to their higher target thresholdvoltages. For example, when programming TFT 416L in vertical NOR string451 b, programming of all other TFTs in vertical NOR strings 451 b and451 a must be inhibited by keeping all their word lines at 0V.

To inhibit further programming or TFT 416L once it has reached itstarget threshold voltage, a half-select voltage (i.e., approximately10V) is applied to local bit line 454. With 10V being placed in thechannel region and 20V being placed on the control gate, only net 10V isapplied across the charge trapping layer, therefore the Fowler-Nordheimtunneling current is insignificant and no meaningful further programmingtakes place on TFT 416L during the remaining sequence of stepped pulsevoltages up to the maximum 20V. By raising the local bit line 454 to 10Vwhile continuing to increment the programming voltage pulses on wordline WL31, all TFTs on vertical NOR strings sharing the same selectedword line are programmed correctly to their higher target thresholdvoltages. The sequence of “program-read-program inhibit” isindispensable for correctly programming tens of thousands TFTs inparallel to their various target threshold voltage states in multilevelcell storage. Absent such program inhibit of individual TFTsover-programming may cause overstepping or merging with the thresholdvoltage of the next higher target threshold voltage state. Although TFT416R and TFT 416L share the same word line, they belong to differentvertical NOR string pairs 452 and 451. It is possible to program bothTFT 416L and TFT 416R in the same programming pulsed voltage sequence,as their respective bit line voltages are supplied through GBL1 and GBL2and are independently controlled. For example, TFT 416L can continue tobe programmed while TFT 416R can be inhibited from further programmingat any time. These program and program-inhibit voltage conditions can bemet because vertical NOR strings 451 a and 451 b of vertical NOR stringpair 491 are controlled by separate word lines 423 p-L and 423 p-Rrespectively, and the voltage on each local bit line can be setindependently from all other vertical NOR string pairs. Duringprogramming, any unselected word line within an addressed word linestack or within unaddressed word line stacks can be brought to 0V,half-select 10 volts, or floated. In the embodiment where global sourceline (e.g., GSL1 of FIG. 4c ) is accessed through a source access selecttransistor (not shown in FIG. 4c ), the access select transistor is offduring programming, resulting in the voltage on local source line 455following the voltage on local bit line 454 during program and programinhibit. The same is true for the embodiment where the voltage on thelocal source line is provided by its parasitic capacitance C representedby capacitor 460 in FIG. 4c . In the embodiment of FIG. 4c , where thereis a global source line but not a source access select transistor, thevoltage applied to the global source line 413-1 of the addressed stringshould preferably track the voltage of the addressed global bit line414-1 during program and program-inhibit.

Each of the incrementally higher voltage programming pulses is followedby a read cycle to determine if TFTs 416L and 416R have reached theirrespective target threshold voltage. If so, the drain, source and bodyvoltages are raised to 10V (alternatively, these voltages are floated toclose to 10V) to inhibit further programming, while word line WL31continues to program other addressed TFTs on the same plane that havenot yet attained their target threshold voltages. This sequenceterminates when all addressed TFTs have been read-verified to becorrectly programmed. In the case of MLC, programming of one of themultiple threshold voltage states can be accelerated by setting eachaddressed global bit line to one of several predetermined voltages(e.g., 0V, 1.5V, 3.0V, or 4.5V, representing the four distinct states ofthe 2-bit data to be stored), and then applying the stepped programmingpulses (up to around 20V) to word line WL31. In this manner, theaddressed TFT receives a predetermined one of the effective tunnelingvoltages (i.e., 20, 18.5, 17, and 15.5 volts, respectively), resultingin one of predetermined threshold voltages being programmed into a TFTin a single programming sequence. Fine programming pulses may besubsequently provided at the individual TFT level.

Accelerated Whole-Plane Parallel Programming

Because of the parasitic capacitance C intrinsic to every local sourceline in a multi-gate vertical NOR string array, all local source linesin a multi-gate vertical NOR string array can have 0V (for program) or10V (for inhibit) momentarily placed (e.g., through global bit line GBL1and bit line access string select transistor 411 and pre-chargetransistor 470) on all vertical NOR strings in advance of applying thehigh voltage pulsing sequence. This procedure may be carried out byaddressing the word line planes plane-by-plane. For each addressed wordline plane, the programming pulsing sequence may be applied to many orall word lines on the addressed word line plane, while holding all wordlines on the other word line planes at 0V, so as to program in parallela large number of TFTs on the addressed plane, followed by individualread-verify, and where necessary, resetting the local source line of aproperly programmed TFT into program-inhibit voltage. This approachprovides a significant advantage, as programming time is relatively long(i.e., around 100 microsecond), while pre-charging all local source linecapacitors or read-verifying all TFTs sharing the addressed word lineplane is more than 1,000 times faster. Therefore, it pays to parallelprogram as many TFTs as possible in each word line plane. Thisaccelerated programming feature provides even greater advantage in MLCprogramming which is considerably slower than single bit programming

Erase Operation

For some charge-trapping materials, the erase operation is performed byreverse-tunneling of the trapped charge, which can be rather slow,sometimes requiring tens of milliseconds of 20V or higher pulsing.Therefore, the erase operation may be implemented at the vertical NORstring array level (“block erase”), often performed in the background. Atypical vertical NOR string array may have 64 word line planes, witheach word line plane controlling, for example, 16,384×16,384 TFTs, for atotal of approximately seventeen billion TFTs. A one-terabit chip maytherefore include approximately 30 such vertical NOR string arrays, iftwo bits of data are stored on each TFT. In some embodiments, blockerase may be carried out by applying around 20V to the P− channel sharedby all TFTs in a vertical NOR string (e.g., body connection 456 in FIG.4c and contact 556 in FIG. 5a ), while holding all word lines in theblock at 0V. The duration of the erase pulse should be such that mostTFTs in the block are erased to a slight enhancement mode thresholdvoltage, i.e., between zero and one volt. Some TFTs will overshoot andbe erased into depletion mode (i.e., a slightly negative thresholdvoltage). A soft programming may be required to return the over-erasedTFTs back into a slight enhancement mode threshold voltage after thetermination of the erase pulses, as part of the erase command VerticalNOR strings that may include one of more depletion mode TFTs that cannotbe programmed into enhancement mode may have to be retired, to bereplaced by spare strings.

Alternatively, rather than providing the erase pulses to the body (i.e.,the P− layer), the local source lines and the local bit lines (e.g.,local source line 455 and local bit line 454 in FIG. 4c ) on allvertical NOR string pairs in the vertical NOR string array are raised toaround 20V, while holding all word lines on all word line planes at 0Vfor the duration of the erase pulse. This scheme requires that theglobal source line and the global bit line select decoders employ highvoltage transistors that can withstand the 20V at their junctions.Alternatively, all TFTs sharing an addressed word line plane can beerased together by applying −20V pulses to all word lines on theaddressed plane, while holding word lines on all other planes at 0V. Allother voltages in the vertical NOR string pairs are held at 0V. Thiswill erase only the X-Y slice of all TFTs touched by the one addressedplane of word lines.

Semi Non-Volatile NOR TFT Strings

Some charge-trapping materials (e.g., oxide-nitride-oxide or “ONO”)suitable for use in the vertical NOR string have long data retentiontime, typically in the order of many years, but relatively low endurance(i.e., performance degrades after some number of write-erase cycles,typically of the order of ten thousand cycles or less). However, in someembodiments one may select charge-trapping materials that store chargefor much reduced retention times, but with much increased endurances(e.g., retention times in order of minutes or hours, endurance in theorder of tens of millions of write-erase cycles). For example, in theembodiment of FIG. 7c , the tunnel dielectric layer 732 c, typically a6-8 nanometer layer of SiO₂, can be reduced in thickness to around 2nanometers, or be replaced by another dielectric material (e.g., SiN) ofsimilar thickness. The much thinner dielectric layer makes possible theuse of modest voltages to introduce electrons by direct tunneling (asdistinct from Fowler-Nordheim tunneling, which requires a highervoltage) into the charge-trapping layer, where they will be trapped froma few minutes to a few hours or days. Charge-trapping layer 732 b can besilicon nitride, conductive nanodots dispersed in a thin dielectricfilm, or a combination of other charge-trapping films, includingisolated thin floating gates. Blocking layer 732 a can be silicondioxide, aluminum oxide, hafnium oxide, silicon nitride, a highdielectric constant dielectric, or any combination thereof. Blockinglayer 732 a blocks electrons in charge-trapping layer 732 b fromescaping to the control gate word line. Trapped electrons willeventually leak out back into active region 730R, either as a result ofthe breakdown of the ultra-thin tunnel dielectric layer, or by reversedirect tunneling. However, such loss of trapped electrons is relativelyslow. One may also use other combinations of charge storage materials,resulting in a high endurance but low retention “semi-volatile” storageTFT that requires periodic write or read refresh operations to replenishthe lost charge. Because the vertical NOR strings of the presentinvention have a relatively fast read access (i.e. low latency), theymay be used in some applications that currently require the use ofdynamic random access memories (DRAMs). The vertical NOR strings of thepresent invention have significant advantages over DRAMs, having a muchlower cost-per-bit, as DRAMs cannot be built in three dimensionalstacks, and having a much lower power dissipation, as the refresh cyclesneed only be run approximately once every few minutes or every fewhours, as compared to every few milliseconds required to refresh DRAMs.The three-dimensional semi-volatile storage TFTs of the presentinvention are achieved by selecting an appropriate material, such asthose discussed above, for the charge-trapping material and byappropriately adapting the program/read/program-inhibit/erase conditionsand incorporating the periodic data refreshes.

NROM/Mirror Bit NOR TFT Strings

In another embodiment of the current invention, the vertical NOR stringsmay be programmed using a channel hot-electron injection approach,similar to that which is used in two-dimensional NROM/Mirror Bittransistors, known to a person skilled in the art. Using the embodimentof FIG. 4a as an example, programming conditions for channelhot-electron injection may be: 8V on control gate 423 p, 0V on localsource line 455 and 5V on local drain line 454. Charge representing onebit is stored in the charge storage layer at one end of channel region456 next to the junction with local bit line 454. By reversing polarityof local source line 455 and local bit line 454, charge representing asecond bit is programmed and stored in the charge storage layer at theopposite end of channel region 456 next to the junction with localsource line 455. Reading both bits requires reading in reverse order ofthe programming, as is well known to those skilled in the art. Channelhot-electron programming is much less efficient than programming bydirect tunneling or Fowler-Nordheim tunneling and therefore it does notlend itself to the massively parallel programming possible withtunneling. However, each TFT has twice the bit density, making itattractive for applications such as archival memory. Erase for the NROMTFT embodiment can be achieved by employing the conventional NROM erasemechanism of band to band tunneling-induced hot-hole injection toneutralize the charge of the trapped electrons: apply −5V on the wordline, 0V to local source line 455 and 5V to local bit line 454.Alternatively, the NROM TFT can be erased by applying a high positivesubstrate voltage V_(bb) to body region 456 with the word line at 0V.Because of the high programming current attendant to channel hotelectron injection programming, all embodiments of vertical NROM TFTstrings must employ hard-wired local source line and local bit line,such as in the embodiments of FIGS. 3a and 6 c.

The above detailed description is provided to illustrate specificembodiments of the present invention and is not intended to be limiting.Numerous variations and modification within the scope of the presentinvention are possible. The present invention is set forth in theaccompanying claims.

I claim:
 1. A memory structure, comprising: a semiconductor substrate having a substantially planar surface and including circuitry formed therein for memory circuit operation; an active column of semiconducting material formed above the semiconductor substrate, the active column extending along a first direction substantially orthogonal to the planar surface of semiconductor substrate and including a first heavily doped region, a second heavily doped region, and a first lightly doped region adjacent and in contact with both the first and second heavily doped regions; charge-trapping material adjacent the first lightly doped region; and a first set of word line conductors provided adjacent the active column, each electrically isolated from another and each extending lengthwise along a second direction that is substantially parallel the planar surface, wherein the first and second heavily doped regions of the active column, the first lightly doped region of the active column, the charge-trapping material and each the word line conductor together form a source region, a drain region, a channel region, a storage region, and a gate electrode of one of a plurality of variable-threshold thin-film transistors along a first NOR memory string, the first and second heavily doped regions forming a common local bit line and a common source region, respectively, of the first NOR memory string.
 2. The memory structure of claim 1, wherein the active column is one of a plurality of active columns arranged in a two-dimensional array having rows of active columns extending along the second direction and rows of active columns extending along a third direction, the second direction and the third direction each being parallel to the planar surface of the semiconductor substrate.
 3. The memory structure of claim 2, further comprising a first set of interconnection conductors, each interconnection conductor running lengthwise along the second direction above or beneath the active columns, wherein the common local bit line of the first NOR memory string is selectably connected to an associated one of the first set of interconnection conductors.
 4. The memory structure of claim 3, further comprising a second set of interconnection conductors, each interconnection conductor running lengthwise along the second direction above or beneath the active columns, wherein the common source region of the first NOR memory string is selectably connected to an associated one of the second set of interconnection conductors.
 5. The memory structure of claim 3, wherein the first set of interconnection conductors comprise a plurality of bit line segments each associated with a predetermined number of active columns within a row of active columns along the second direction and wherein each bit line segment is selectably electrically isolated from other bit line segments of the row of active columns, and wherein each bit line segment is electrically connected to the common local bit lines of its associated active columns.
 6. The memory structure of claim 5, further comprising a plurality of segment-select transistors each selectably connecting a corresponding bit line segment to the circuitry in the semiconductor substrate.
 7. The memory structure of claim 6, wherein the circuitry in the semiconductor substrate comprises at least a plurality of sense amplifiers distributed throughout the planar surface of the semiconductor substrate, the sense amplifiers being each connected by a different group of one or more segment-select transistors to corresponding bit line segments.
 8. The memory structure of claim 6, wherein the segment-select transistors are formed in the semiconductor substrate.
 9. The memory structure of claim 5, wherein the first set of interconnection conductors further comprise a plurality of regional bit line segments, each regional bit line segment being selectably connected to a plurality of bit line segments.
 10. The memory structure of claim 9, further comprising a second set of word line conductors, each electrically isolated from another and each extending lengthwise along the second direction, wherein the active column further comprises a second lightly doped region also adjacent and in contact with the charge-trapping material and both the first and second heavily doped regions, such that the first and second heavily doped regions of the active column, the second lightly doped region of the active column, the charge-trapping material and each the second set of word line conductors together form a source region, a drain region, a channel region, a storage region, and a gate electrode of one of a plurality of variable-threshold thin-film transistors along a second NOR memory string, the first and second heavily doped regions forming a local bit line and a common source region, respectively, of the second NOR memory string.
 11. The memory structure of claim 10, wherein each word line conductor of the first and second sets word line conductor provides gate terminals for the variable-threshold thin-film transistors in active columns on opposite sides of the word line conductor, and wherein the local bit lines in adjacent active columns on opposite side of the word line conductor are associated with different bit line segments.
 12. The memory structure of claim 10, wherein the second set of interconnection conductors comprise a plurality of source line segments each associated with a predetermined number of active columns within a row of active columns along the second direction and wherein each source line segment is selectably electrically isolated from other source line segments of the row of active columns, and wherein each source line segment is electrically connected to the common source regions of its associated active columns.
 13. The memory structure of claim 12, further comprising a global source line and a plurality of segment-select transistors, each segment-select transistor connecting a predetermined number of source line segments to the global source line.
 14. The memory structure of claim 13, further comprising in each active column a pre-charge transistor for electrically connecting the common source region of the active column to the common local bit line of the active column.
 15. The memory structure of claim 14, wherein the common source regions that are connected by each source line segment provide a capacitance that serves as a virtual voltage source during a read, programming, program-inhibit or erase operation on one or more variable-threshold thin-film transistors in the active columns associated with the source line segment.
 16. The memory structure of claim 3, wherein the circuitry comprises a body bias voltage source, wherein the first lightly doped region of each active column is connected to the body bias voltage source by a conductor in the first set of interconnection conductors.
 17. The memory structure of claim 12 further comprising a plurality of charging columns each being associated with one of the source line segments, each charging column having first and second heavily doped regions structurally substantially the same as the first and second heavily doped regions of one of the active columns, wherein each charging column further comprises a plurality of pass transistors each selectably connecting the first and second heavily doped regions of the charging column, and wherein at least one of the first and second heavily doped regions is connected to the circuitry in the semiconductor substrate.
 18. The memory structure of claim 17, wherein each charging column is formed between two neighboring source line segments.
 19. The memory structure of claim 17, wherein one or more of the first set of word line conductors activate one or more of the pass transistors in one or more of the charging columns.
 20. The memory structure of claim 17, wherein each charging column is connected to a voltage source in the semiconductor substrate by a segment-select transistor in the semiconductor substrate.
 21. The memory structure of claim 20, wherein the voltage source supplies an erase voltage for a common source region during an erase operation.
 22. The memory structure of claim 2, wherein the active columns are isolated from each other by an isolation dielectric material or by an air gap.
 23. The memory structure of claim 1, wherein the first set of word line conductors in are insulated from each other by an isolation dielectric material or air gap.
 24. The memory structure of claim 3, wherein the first set of word line conductors at different positions along the first direction form steps of a stair-case structure and wherein each word line conductor is connected at the stair-case structure by vias to a corresponding one of the first set of interconnection conductors.
 25. A composite memory structure comprising first and second memory structures provided one on top of the other, wherein each modular memory structure comprises the memory structure of claim
 24. 26. The composite memory structure of claim 25, wherein the first and second modular memory structures are isolated from each other by a dielectric layer.
 27. The memory structure of claim 26, wherein active columns in the first and second modular memory structures are aligned along the first direction and wherein in the common source regions of corresponding active columns are connected by vias through the dielectric layer.
 28. The memory structure of claim 2, further comprising metallic pylons embedded in the common source regions and the common local bit lines of the active columns.
 29. The memory structure of claim 28, wherein the metallic pylons each comprises one or more of titanium nitride, tungsten nitride or tungsten.
 30. The memory structure of claim 29, wherein each metallic pylon is formed using an atomic layer deposition technique.
 31. The memory structure of claim 1, wherein the first lightly doped region in the active column comprises a first section and a second section, and wherein the first section of the first lightly doped region serves as channel regions for the variable-threshold thin-film transistors in the first NOR memory string and wherein the second section has a dopant concentration multiple times that of the first section. 